mfmv should check intra_only for reference frames

This is correct in the spec, but the code wasn't doing the check.

BUG=aomedia:1405

Change-Id: I6e22f78d724583ad4cfdbbffc15e61b636810f76

av1/common/mvref_common.c

diff --git a/av1/common/mvref_common.c b/av1/common/mvref_common.c
index 67c26c0..5a50710 100644
--- a/av1/common/mvref_common.c
+++ b/av1/common/mvref_common.c
@@ -1638,6 +1638,8 @@
   int ref_frame_idx = cm->frame_refs[FWD_RF_OFFSET(ref_frame)].idx;
   if (ref_frame_idx < 0) return 0;
 
+  if (cm->buffer_pool->frame_bufs[ref_frame_idx].intra_only) return 0;
+
   if (cm->buffer_pool->frame_bufs[ref_frame_idx].mi_rows != cm->mi_rows ||
       cm->buffer_pool->frame_bufs[ref_frame_idx].mi_cols != cm->mi_cols)
     return 0;