Set arf showable to 0 when not using show existing

STATS_CHANGED

Change-Id: I14f918ae5a8a077055fd7381dc12ad59e01aedce
diff --git a/av1/encoder/encode_strategy.c b/av1/encoder/encode_strategy.c
index a7f8d6a..a8f153d 100644
--- a/av1/encoder/encode_strategy.c
+++ b/av1/encoder/encode_strategy.c
@@ -786,6 +786,8 @@
             tf_buf, &frame_diff, q_index, cm->seq_params->bit_depth);
         if (show_existing_alt_ref) {
           cpi->common.showable_frame |= 1;
+        } else {
+          cpi->common.showable_frame = 0;
         }
       }
       if (gf_group->frame_type[cpi->gf_frame_index] != KEY_FRAME) {