vc1dec: Fix tff == 0 handling in init_block_index()

This fixes several files from VLC ticket 5887.
Signed-off-by: NMartin Storsjö <martin@martin.st>

libavcodec/vc1dec.c

@@ -77,7 +77,7 @@ static void init_block_index(VC1Context *v)
 {
     MpegEncContext *s = &v->s;
     ff_init_block_index(s);
-    if (v->field_mode && v->second_field) {
+    if (v->field_mode && !(v->second_field ^ v->tff)) {
         s->dest[0] += s->current_picture_ptr->f.linesize[0];
         s->dest[1] += s->current_picture_ptr->f.linesize[1];
         s->dest[2] += s->current_picture_ptr->f.linesize[2];