This matches how its done for SPS/PPS.
An alternative to this is to check it when its used.

Fixes null pointer dereference
Fixes: signal_sigsegv_e30a43_1437_CIP_A_Panasonic_3.bit
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>