vc1: Fix pic_header_flag=0 (SA10143.vc1)

Bug introduced in:
commit 4509be3d
Author: Michael Niedermayer <michaelni@gmx.at>
Date:   Tue Oct 11 11:56:42 2011 +0200

    vc1: reset interlaced variables, prevent another bunch of crashes.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vc1.c

@@ -825,10 +825,10 @@ int vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
     int scale, shift, i; /* for initializing LUT for intensity compensation */
 
     v->numref=0;
-    v->fcm=0;
-    v->field_mode=0;
     v->p_frame_skipped = 0;
     if (v->second_field) {
+        if(v->fcm!=2 || v->field_mode!=1)
+            return -1;
         v->s.pict_type = (v->fptype & 1) ? AV_PICTURE_TYPE_P : AV_PICTURE_TYPE_I;
         if (v->fptype & 4)
             v->s.pict_type = (v->fptype & 1) ? AV_PICTURE_TYPE_BI : AV_PICTURE_TYPE_B;