demux: fix backward demuxing not grabbing all audio packets

The previous commit broke audio playback (maybe this is what 4. was
about?). But it wasn't the fault of the commit; it just exposed
pre-existing issues.

If the packet queue search can't get all packets, it checked
queue->is_bof to see whether there could be previous packets. But
obviously, is_bof can be set, even if the search start packet wasn't the
first one.

This was especially observable with audio, because audio by default
needs preroll packets, and plays artifacts if they're missing.

Fix by using the BOF playback condition for this purpose too.