avcodec/h264: Undefined behavior (left shift of 12852653 by 8 places cannot be represented in type 'int')

Fixes: asan_heap-oob_84f75d_8_asan_heap-oob_a2a00a_341_mbc.ts

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