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>

libavcodec/vc1.c

@@ -629,6 +629,8 @@ int ff_vc1_parse_frame_header(VC1Context *v, GetBitContext* gb)
 {
     int pqindex, lowquant, status;
 
+    v->field_mode = 0;
+    v->fcm = 0;
     if (v->finterpflag)
         v->interpfrm = get_bits1(gb);
     if (!v->s.avctx->codec)