vc1: don't read the interpfrm and bfraction elements for interlaced frames

This matches the spec as well as the reference decoder, and fixes a bug
with interlaced frame decoding.
Signed-off-by: Kostya Shishkov <kostya.shishkov@gmail.com>

libavcodec/vc1.c

@@ -911,6 +911,7 @@ int vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
         }
         goto parse_common_info;
     }
+    if (v->fcm == PROGRESSIVE) {
         if (v->finterpflag)
             v->interpfrm = get_bits1(gb);
         if (v->s.pict_type == AV_PICTURE_TYPE_B) {
@@ -920,6 +921,7 @@ int vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
                 v->s.pict_type = AV_PICTURE_TYPE_BI; /* XXX: should not happen here */
             }
         }
+    }
 
     parse_common_info:
     if (v->field_mode)