avcodec/h264: Only reinit quant tables if a new PPS is allowed

Fixes null pointer dereference
Fixes: signal_sigsegv_3042097_3007_cov_1741463594_non_monotone_timestamps1.mkv

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