Fix videoplayer crash on quit

The sound stream thread was trying to update the stream during/after destruction of the video state.

apps/openmw/mwrender/videoplayer.cpp

@@ -1099,6 +1099,8 @@ void VideoState::deinit()
 {
     this->quit = true;
 
+    this->AudioTrack.reset();
+
     this->audioq.cond.notify_one();
     this->videoq.cond.notify_one();