avcodec/mmvideo: Bounds check 2nd line of HHV Intra blocks

Fixes out of array access
Fixes: asan_heap-oob_4da4f3_8_asan_heap-oob_4da4f3_419_scene1a.mm

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>