avcodec/h264: use subsample factors of the used pixel format

Fixes out of array read
Fixes: 1cb91c36c4e55463f14aacb9bdf55b38-asan_heap-oob_106cbce_5617_cov_11212800_h264_mmx_chroma_intra_lf.mp4
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
