Resolve codec issue between tile-groups and ext-refs

This fixes the coding failure when both tile-groups and ext-refs
are turned on. It clears the nightly-run unit test failure.

Change-Id: Idb67580ebe3ef625cf3ab18dcf7a274db4dacab3

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index e4f464b..b47835f 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -4061,6 +4061,13 @@
 #if CONFIG_TILE_GROUPS
   write_uncompressed_header(cpi, wb);
 
+#if CONFIG_EXT_REFS
+  if (cm->show_existing_frame) {
+    total_size = aom_wb_bytes_written(wb);
+    return (uint32_t)total_size;
+  }
+#endif  // CONFIG_EXT_REFS
+
   // Write the tile length code
   tile_size_bytes_wb = *wb;
   aom_wb_write_literal(wb, 3, 2);