MKV: fix playback of AC-3 with bogus default duration
MKV: fix playback of AC-3 with bogus default duration

Fixes #8512
The fix is similar to that one in ffmpeg/libav http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavformat/matroskadec.c;#l2813

Signed-off-by: Jean-Baptiste Kempf <7b85a41a628204b76aba4326273a3ccc74bd009a@videolan.org>

vlc-mirror/vlc,krichter722/vlc,shyamalschandra/vlc,vlc-mirror/vlc,shyamalschandra/vlc,vlc-mirror/vlc,vlc-mirror/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc,krichter722/vlc,vlc-mirror/vlc,xkfz007/vlc,xkfz007/vlc,shyamalschandra/vlc,krichter722/vlc,vlc-mirror/vlc,xkfz007/vlc,xkfz007/vlc,krichter722/vlc,krichter722/vlc,shyamalschandra/vlc,shyamalschandra/vlc,krichter722/vlc,shyamalschandra/vlc,shyamalschandra/vlc