RenderCallbackSPDIF might call read_buffer with NULL data. The purpose
is to drain data from the buffer when the output is muted. Add a check
to call av_fifo_drain() in this case.
git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@34241 b3059339-0415-0410-9bf9-f77b7e298cf2
git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@34242 b3059339-0415-0410-9bf9-f77b7e298cf2