vc1dec: Fix current ptr selection in vc1_mc_4mv_chroma()

No sample tried shows a difference.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/vc1dec.c

@@ -380,7 +380,7 @@ static void vc1_mc_1mv(VC1Context *v, int dir)
         uvmy = uvmy + ((uvmy < 0) ? (uvmy & 1) : -(uvmy & 1));
     }
     if (!dir) {
-        if (v->field_mode && (v->cur_field_type != v->ref_field_type[dir]) && v->cur_field_type) {
+        if (v->field_mode && (v->cur_field_type != v->ref_field_type[dir]) && v->second_field) {
             srcY = s->current_picture.f.data[0];
             srcU = s->current_picture.f.data[1];
             srcV = s->current_picture.f.data[2];
@@ -554,7 +554,7 @@ static void vc1_mc_4mv_luma(VC1Context *v, int n, int dir)
     my = s->mv[dir][n][1];
 
     if (!dir) {
-        if (v->field_mode && (v->cur_field_type != v->ref_field_type[dir]) && v->cur_field_type) {
+        if (v->field_mode && (v->cur_field_type != v->ref_field_type[dir]) && v->second_field) {
             srcY = s->current_picture.f.data[0];
         } else
             srcY = s->last_picture.f.data[0];
@@ -829,7 +829,7 @@ static void vc1_mc_4mv_chroma(VC1Context *v, int dir)
     }
 
     if (!dir) {
-        if (v->field_mode && (v->cur_field_type != chroma_ref_type) && v->cur_field_type) {
+        if (v->field_mode && (v->cur_field_type != chroma_ref_type) && v->second_field) {
             srcU = s->current_picture.f.data[1];
             srcV = s->current_picture.f.data[2];
         } else {