vc1: Reset numref if fieldmode is not set

There are samples in the wild with B-frames and P-frames with different
interlace mode.

CC: libav-stable@libav.org
Reported-by: Jean-Baptiste Kempf <jb@videolan.org>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/vc1.c

@@ -1005,6 +1005,8 @@ 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);