Fix support for mixed-protocol external subtitles

MediaBrowser.MediaEncoding/Subtitles/SubtitleEncoder.cs

@@ -172,7 +172,7 @@ namespace MediaBrowser.MediaEncoding.Subtitles
                 inputFiles = new[] { mediaSource.Path };
             }
 
-            var fileInfo = await GetReadableFile(mediaSource.Path, inputFiles, mediaSource.Protocol, subtitleStream, cancellationToken).ConfigureAwait(false);
+            var fileInfo = await GetReadableFile(mediaSource.Path, inputFiles, _mediaSourceManager.GetPathProtocol(subtitleStream.Path), subtitleStream, cancellationToken).ConfigureAwait(false);
 
             var stream = await GetSubtitleStream(fileInfo.Path, fileInfo.Protocol, fileInfo.IsExternal, cancellationToken).ConfigureAwait(false);