h264: check that execute_decode_slices() is not called too many times

Fixes invalid reads.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org
h264: reject mismatching luma/chroma bit depths during sps parsing

There is no point in delaying the check and it avoids bugs with a
half-initialized context.

Fixes invalid reads.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org