The test stream sometimes causes the ASF demuxer to report "unknown GUID"
several times then act as if the headers have been received, with the audio
output thread waiting forever for data to be passed to it by the decoder, in
this case the ffmpeg audio decoder.
This particular hang occurs if playback is stopped before the demuxer has
decided that headers have been received (later, and we'd have the problem
which the parent cset of this cset fixes); having the demuxer control insert
empty audio buffers where it would otherwise wait forever for the audio
output thread to receive some data.
Test stream: mmsh://213.92.19.8:80/radiodeejay?MSWMExt=.asf