avcodec/utils: set AVFrame format unconditional

Fixes inconsistency and out of array accesses
Fixes: 10cdd7e63e7f66e3e66273939e0863dd-asan_heap-oob_1a4ff32_7078_cov_4056274555_mov_h264_aac__mp4box_frag.mp4
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>