vp10: don't allow comp_inter_inter on keyframes.

Change-Id: Ibd0e13721a2bb71c532d20b36c42f4cccf5c5de2

vp10/decoder/decodeframe.c

@@ -47,6 +47,8 @@
 
 static int is_compound_reference_allowed(const VP10_COMMON *cm) {
   int i;
+  if (frame_is_intra_only(cm))
+    return 0;
   for (i = 1; i < REFS_PER_FRAME; ++i)
     if (cm->ref_frame_sign_bias[i + 1] != cm->ref_frame_sign_bias[1])
       return 1;

vp10/encoder/encodeframe.c

@@ -2807,6 +2807,8 @@ void vp10_encode_frame(VP10_COMP *cpi) {
       cm->comp_var_ref[0] = LAST_FRAME;
       cm->comp_var_ref[1] = GOLDEN_FRAME;
     }
+  } else {
+    cpi->allow_comp_inter_inter = 0;
   }
 
   if (cpi->sf.frame_parameter_update) {