Condidtionally drop projection from the alt2 frame

Change-Id: I3ac81379669678254672f125df281980b687e16e

av1/common/mvref_common.c

@@ -1931,7 +1931,10 @@ void av1_setup_motion_field(AV1_COMMON *cm) {
   }
 
   motion_field_projection(cm, ALTREF_FRAME);
-  motion_field_projection(cm, ALTREF2_FRAME);
+
+  if (alt2_frame_index > cur_frame_index)
+    motion_field_projection(cm, ALTREF2_FRAME);
+
   motion_field_projection(cm, BWDREF_FRAME);
 }
 #endif  // CONFIG_MFMV