oma: refactor seek function

Properly propagate seek errors from avio and the generic pcm seek.
oma: check geob tag boundary

Prevent read after buffer boundary on corrupted tag.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
oma: correctly mark and decrypt partial packets

Incomplete crypted files would lead to a read after buffer boundary
otherwise.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org