Fixes out of array read
Fixes: asan_static-oob_30328b6_719_cov_3325483287_H264_artifacts_motion.h264

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 69aa7936)

Conflicts:

	libavcodec/h264_ps.c
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>