avcodec/vc1: FIELDTX is only present in interlaced frame I/BI pictures

If v->fieldtx_is_raw is not reset to zero, it may spill over from a previous
interlaced frame I/BI picture.

Signed-off-by: Jerome Borsboom <jerome.borsboom@carpalis.nl>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vc1.c

@@ -1010,7 +1010,8 @@ int ff_vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
                 return -1;
             av_log(v->s.avctx, AV_LOG_DEBUG, "FIELDTX plane encoding: "
                    "Imode: %i, Invert: %i\n", status>>1, status&1);
-        }
+        } else
+            v->fieldtx_is_raw = 0;
         status = bitplane_decoding(v->acpred_plane, &v->acpred_is_raw, v);
         if (status < 0)
             return -1;