Such changes are forbidden in H.264 and lead to race conditions

Fixes out of array read
Fixes: signal_sigsegv_f9796a_1613_cov_3114610371_FM1_BT_B.h264
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>