demux_mkv: seek: fix bogus audio packet from earlier position
Due to a bug created back in 2006 when SimpleBlock support was added,
demux_mkv demuxed one audio packet from the initial file position
after a seek, then skipped the following ones until a video keyframe
was found. This wasn't very noticeable earlier, but it had bad effects
after the recently added -initial-audio-sync code as the extra packet
with an earlier timestamp confused timing calculations and resulted in
desync after seeking. Fix.