vc1: Enable the interlaced B-frame codepath.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/vc1.c

@@ -1188,7 +1188,6 @@ int ff_vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
             if (v->bfraction == 0) {
                 return -1;
             }
-            return -1; // This codepath is still incomplete thus it is disabled
         }
         if (v->extended_mv)
             v->mvrange = get_unary(gb, 0, 3);