avcodec/mpeg2dec: Fix field selection for skipped macroblocks

For B field pictures, the spec says,

> The prediction shall be made from the field of the same parity as the field being predicted.

I did it.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg12dec.c

@@ -1969,6 +1969,8 @@ static int mpeg_decode_slice(MpegEncContext *s, int mb_y,
                     s->mv[0][0][1] = s->last_mv[0][0][1];
                     s->mv[1][0][0] = s->last_mv[1][0][0];
                     s->mv[1][0][1] = s->last_mv[1][0][1];
+                    s->field_select[0][0] = (s->picture_structure - 1) & 1;
+                    s->field_select[1][0] = (s->picture_structure - 1) & 1;
                 }
             }
         }