Fix exceptions while scanning

Fixes these exceptions:
```
[2019-12-22 20:48:14.779 +01:00] [ERR] Error in WaitForExit
System.InvalidOperationException: No process is associated with this object.
   at System.Diagnostics.Process.EnsureState(State state)
   at System.Diagnostics.Process.EnsureState(State state)
   at System.Diagnostics.Process.GetWaitState()
   at System.Diagnostics.Process.WaitForExitCore(Int32 milliseconds)
   at System.Diagnostics.Process.WaitForExit(Int32 milliseconds)
   at Emby.Server.Implementations.Diagnostics.CommonProcess.WaitForExit(Int32 timeMs) in /home/pi/dev/jellyfin/Emby.Server.Implementations/Diagnostics/CommonProcess.cs:line 100
   at MediaBrowser.MediaEncoding.Encoder.MediaEncoder.StopProcess(ProcessWrapper process, Int32 waitTimeMs) in /home/pi/dev/jellyfin/MediaBrowser.MediaEncoding/Encoder/MediaEncoder.cs:line 785
[2019-12-22 20:48:14.790 +01:00] [INF] Killing ffmpeg process
[2019-12-22 20:48:14.795 +01:00] [ERR] Error killing process
System.InvalidOperationException: No process is associated with this object.
   at System.Diagnostics.Process.EnsureState(State state)
   at System.Diagnostics.Process.EnsureState(State state)
   at System.Diagnostics.Process.Kill()
   at Emby.Server.Implementations.Diagnostics.CommonProcess.Kill() in /home/pi/dev/jellyfin/Emby.Server.Implementations/Diagnostics/CommonProcess.cs:line 95
   at MediaBrowser.MediaEncoding.Encoder.MediaEncoder.StopProcess(ProcessWrapper process, Int32 waitTimeMs) in /home/pi/dev/jellyfin/MediaBrowser.MediaEncoding/Encoder/MediaEncoder.cs:line 799
[2019-12-22 20:48:14.808 +01:00] [ERR] Error in "ffprobe"
System.Text.Json.JsonException: The JSON value could not be converted to System.String. Path: $.streams[0].start_pts | LineNumber: 32 | BytePositionInLine: 26.
 ---> System.InvalidOperationException: Cannot get the value of a token type 'Number' as a string.
   at System.Text.Json.Utf8JsonReader.GetString()
   at System.Text.Json.Serialization.Converters.JsonConverterString.Read(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options)
   at System.Text.Json.JsonPropertyInfoNotNullable`4.OnRead(JsonTokenType tokenType, ReadStack& state, Utf8JsonReader& reader)
   at System.Text.Json.JsonPropertyInfo.Read(JsonTokenType tokenType, ReadStack& state, Utf8JsonReader& reader)
   at System.Text.Json.JsonSerializer.HandleValue(JsonTokenType tokenType, JsonSerializerOptions options, Utf8JsonReader& reader, ReadStack& state)
   at System.Text.Json.JsonSerializer.ReadCore(JsonSerializerOptions options, Utf8JsonReader& reader, ReadStack& readStack)
   --- End of inner exception stack trace ---
   at System.Text.Json.ThrowHelper.ReThrowWithPath(ReadStack& readStack, Utf8JsonReader& reader, Exception ex)
   at System.Text.Json.JsonSerializer.ReadCore(JsonSerializerOptions options, Utf8JsonReader& reader, ReadStack& readStack)
   at System.Text.Json.JsonSerializer.ReadCore(JsonReaderState& readerState, Boolean isFinalBlock, ReadOnlySpan`1 buffer, JsonSerializerOptions options, ReadStack& readStack)
   at System.Text.Json.JsonSerializer.ReadAsync[TValue](Stream utf8Json, Type returnType, JsonSerializerOptions options, CancellationToken cancellationToken)
   at MediaBrowser.MediaEncoding.Encoder.MediaEncoder.GetMediaInfoInternal(String inputPath, String primaryPath, MediaProtocol protocol, Boolean extractChapters, String probeSizeArgument, Boolean isAudio, Nullable`1 videoType, Boolean forceEnableLogging, CancellationToken cancellationToken) in /home/pi/dev/jellyfin/MediaBrowser.MediaEncoding/Encoder/MediaEncoder.cs:line 399
   at MediaBrowser.Providers.MediaInfo.FFProbeVideoInfo.ProbeVideo[T](T item, MetadataRefreshOptions options, CancellationToken cancellationToken) in /home/pi/dev/jellyfin/MediaBrowser.Providers/MediaInfo/FFProbeVideoInfo.cs:line 122
   at MediaBrowser.Providers.Manager.MetadataService`2.RunCustomProvider(ICustomMetadataProvider`1 provider, TItemType item, String logName, MetadataRefreshOptions options, RefreshResult refreshResult, CancellationToken cancellationToken) in /home/pi/dev/jellyfin/MediaBrowser.Providers/Manager/MetadataService.cs:line 815
```

1 files changed, 15 insertions, 15 deletions

diff --git a/MediaBrowser.MediaEncoding/Encoder/MediaEncoder.cs b/MediaBrowser.MediaEncoding/Encoder/MediaEncoder.cs
index 6bcd6cd46..e0f7b992c 100644
--- a/MediaBrowser.MediaEncoding/Encoder/MediaEncoder.cs
+++ b/MediaBrowser.MediaEncoding/Encoder/MediaEncoder.cs
@@ -397,7 +397,8 @@ namespace MediaBrowser.MediaEncoding.Encoder
                 try
                 {
                     result = await JsonSerializer.DeserializeAsync<InternalMediaInfoResult>(
-                                        process.StandardOutput.BaseStream).ConfigureAwait(false);
+                                        process.StandardOutput.BaseStream,
+                                        cancellationToken: cancellationToken).ConfigureAwait(false);
                 }
                 catch
                 {
@@ -406,24 +407,24 @@ namespace MediaBrowser.MediaEncoding.Encoder
                     throw;
                 }
 
-                if (result == null || (result.streams == null && result.format == null))
+                if (result == null || (result.Streams == null && result.Format == null))
                 {
                     throw new Exception("ffprobe failed - streams and format are both null.");
                 }
 
-                if (result.streams != null)
+                if (result.Streams != null)
                 {
                     // Normalize aspect ratio if invalid
-                    foreach (var stream in result.streams)
+                    foreach (var stream in result.Streams)
                     {
-                        if (string.Equals(stream.display_aspect_ratio, "0:1", StringComparison.OrdinalIgnoreCase))
+                        if (string.Equals(stream.DisplayAspectRatio, "0:1", StringComparison.OrdinalIgnoreCase))
                         {
-                            stream.display_aspect_ratio = string.Empty;
+                            stream.DisplayAspectRatio = string.Empty;
                         }
 
-                        if (string.Equals(stream.sample_aspect_ratio, "0:1", StringComparison.OrdinalIgnoreCase))
+                        if (string.Equals(stream.SampleAspectRatio, "0:1", StringComparison.OrdinalIgnoreCase))
                         {
-                            stream.sample_aspect_ratio = string.Empty;
+                            stream.SampleAspectRatio = string.Empty;
                         }
                     }
                 }
@@ -778,6 +779,7 @@ namespace MediaBrowser.MediaEncoding.Encoder
                 _runningProcesses.Add(process);
             }
         }
+
         private void StopProcess(ProcessWrapper process, int waitTimeMs)
         {
             try
@@ -786,18 +788,16 @@ namespace MediaBrowser.MediaEncoding.Encoder
                 {
                     return;
                 }
-            }
-            catch (Exception ex)
-            {
-                _logger.LogError(ex, "Error in WaitForExit");
-            }
 
-            try
-            {
                 _logger.LogInformation("Killing ffmpeg process");
 
                 process.Process.Kill();
             }
+            catch (InvalidOperationException)
+            {
+                // The process has already exited or
+                // there is no process associated with this Process object.
+            }
             catch (Exception ex)
             {
                 _logger.LogError(ex, "Error killing process");