h264: check the pixel format directly and force a reinit on mismatches.

The existing checks are insufficient to detect a pixel format
changes in case of some damaged streams.
Fixes inconsistency and later out of array accesses

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