avcodec/utils: Align dimensions by at least their chroma sub-sampling factors.

Fixes: out of array accesses
Fixes: asan_heap-oob_112c6b3_13_012.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
