avcodec/h264: make the first field of H264Context an AVClass

Fixes use of freed memory
Fixes: asan_heap-uaf_3660f67_757_cov_1257014655_Hi422FR1_SONY_A.jsv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>