vc1: Always reset numref when parsing a new frame header.

Fixes an issue where the B-frame coding mode switches from interlaced
fields to interlaced frames, causing incorrect decisions in the motion
compensation code and resulting in visual artifacts.

CC: libav-stable@libav.org
Signed-off-by: Tim Walker <tdskywalker@gmail.com>

libavcodec/vc1.c

@@ -842,6 +842,7 @@ int ff_vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
     int mbmodetab, imvtab, icbptab, twomvbptab, fourmvbptab; /* useful only for debugging */
     int field_mode, fcm;
 
+    v->numref          = 0;
     v->p_frame_skipped = 0;
     if (v->second_field) {
         v->s.pict_type = (v->fptype & 1) ? AV_PICTURE_TYPE_P : AV_PICTURE_TYPE_I;
@@ -1011,8 +1012,6 @@ int ff_vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
                 v->reffield          = get_bits1(gb);
                 v->ref_field_type[0] = v->reffield ^ !v->cur_field_type;
             }
-        } else {
-            v->numref = 0;
         }
         if (v->extended_mv)
             v->mvrange = get_unary(gb, 0, 3);