vc1: always read the bfraction element for interlaced fields

Previously, it would not be read if refdist_flag was not set, however
according to the spec and the reference decoder, it should always be read.

Signed-off-by: Kostya Shishkov <kostya.shishkov@gmail.com>

libavcodec/vc1.c

@@ -894,13 +894,12 @@ int vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
     if (v->field_mode) {
         if (!v->refdist_flag)
             v->refdist = 0;
-        else {
+        else if ((v->s.pict_type != AV_PICTURE_TYPE_B) && (v->s.pict_type != AV_PICTURE_TYPE_BI)) {
-            if ((v->s.pict_type != AV_PICTURE_TYPE_B)
-                && (v->s.pict_type != AV_PICTURE_TYPE_BI)) {
             v->refdist = get_bits(gb, 2);
             if (v->refdist == 3)
                 v->refdist += get_unary(gb, 0, 16);
-            } else {
+        }
+        if ((v->s.pict_type == AV_PICTURE_TYPE_B) || (v->s.pict_type == AV_PICTURE_TYPE_BI)) {
             v->bfraction_lut_index = get_vlc2(gb, ff_vc1_bfraction_vlc.table, VC1_BFRACTION_VLC_BITS, 1);
             v->bfraction           = ff_vc1_bfraction_lut[v->bfraction_lut_index];
             v->frfd = (v->bfraction * v->refdist) >> 8;
@@ -908,7 +907,6 @@ int vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
             if (v->brfd < 0)
                 v->brfd = 0;
         }
-        }
         goto parse_common_info;
     }
     if (v->fcm == PROGRESSIVE) {