h264_refs: make sure not to write over the bounds of the default ref list

Fixes invalid writes.
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org
h263dec: sanitize a condition.

Call ff_mpeg4_decode_picture_header() only when the decoder has the
MPEG4 codec id, not based on a vaguely related value of h263_pred.