Validate AV stream index before accessing stream.

Fixes #528.

(cherry picked from commit d479280)

mythtv/libs/libmythtv/decoders/avformatdecoder.cpp

@@ -4133,7 +4133,9 @@ QString AvFormatDecoder::GetTrackDesc(uint type, uint TrackNo)
     QString forcedString = forced ? QObject::tr(" (forced)") : "";
 
     int av_index = m_tracks[type][TrackNo].m_av_stream_index;
-    AVStream *stream = m_ic->streams[av_index];
+    AVStream *stream { nullptr };
+    if (av_index >= 0 && av_index < m_ic->nb_streams)
+        stream = m_ic->streams[av_index];
     AVDictionaryEntry *entry =
         stream ? av_dict_get(stream->metadata, "title", NULL, 0) : nullptr;
     QString user_title = entry ? QString(R"( "%1")").arg(entry->value) : "";