Re-arrange ref frame priorities

This resolves encoding failures due to certain speed features
assuming the existence of an ARF.

BUG=aomedia:2834

STATS_CHANGED

Change-Id: I5a3d6a679fd33eec31519f17574eda851a52db4a
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 3aa9cab..0a4bce7 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -3628,8 +3628,8 @@
   }
 
   if (cpi->rc.is_src_frame_alt_ref) {
-    if (sf->inter_sf.alt_ref_search_fp) {
-      assert(cpi->ref_frame_flags & av1_ref_frame_flag_list[ALTREF_FRAME]);
+    if (sf->inter_sf.alt_ref_search_fp &&
+        (cpi->ref_frame_flags & av1_ref_frame_flag_list[ALTREF_FRAME])) {
       mask->pred_modes[ALTREF_FRAME] = 0;
       disable_inter_references_except_altref(mask->ref_combo);
       disable_reference(INTRA_FRAME, mask->ref_combo);