qpeg: fix an off by 1 error in the MV check

height - me_y is the line from which we read, so it must be strictly
smaller than the frame height. Fixes possible invalid reads in corrupted
files.

Also, use a proper context for logging the error.

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