Assert positive ref_count in av1_set_frame_refs.

If cm->ref_frame_map[map_idx] is not a null pointer, the RefCntBuffer it
points to should have a positive ref_count.

Change-Id: Ie7c33401c409f2d1d9400d0a3cc3ec89b8f68438

av1/common/mvref_common.c

diff --git a/av1/common/mvref_common.c b/av1/common/mvref_common.c
index 60cb3d1..e58a7bd 100644
--- a/av1/common/mvref_common.c
+++ b/av1/common/mvref_common.c
@@ -1396,7 +1396,9 @@
     ref_frame_info[i].buf = buf;
 
     if (buf == NULL) continue;
-    // TODO(zoeliu@google.com): To verify the checking on ref_count.
+    // If this assertion fails, there is a reference leak.
+    assert(buf->ref_count > 0);
+    // TODO(wtc@google.com): Remove the checking on ref_count after 2019-03-01.
     if (buf->ref_count <= 0) continue;
 
     const int offset = (int)buf->order_hint;