avcodec/h264: update current_sps & sps->new only after the whole slice header decoder and init code finished

This avoids them being cleared before the full initialization finished

Fixes out of array read
Fixes: asan_heap-oob_f0c5e6_7071_cov_1605985132_mov_h264_aac__Demo_FlagOfOurFathers.mov
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>