avcodec/cook: check that the subpacket sizes fit in block_align

Fixes out of array read
Fixes: asan_heap-oob_fb5c50_19_018.rmvb
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
