h264: Add an AVClass pointer to H264Context

Sample-Id: 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>
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>