avcodec/vc1: reset fcm/field_mode in non advanced header parsing

Fixes NULL pointer dereference
Fixes: signal_sigsegv_1ab8bf4_2847_cov_4254117347_SA10091.vc1
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>