Cleanup encode_show_existing_frame logic

This is a followup to comments made post-submit.

Change-Id: Ic58e971b7843b121ed522f5f5989b753b7bdd2dc
diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index f393728..b5f8cb2 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -3495,7 +3495,7 @@
     // If we are not indicating to the decoder that this frame is
     // a show_existing_frame, which occurs in error_resilient mode,
     // we still want to refresh the LAST_FRAME.
-    cpi->refresh_last_frame = encode_show_existing_frame(cm) ? 0 : 1;
+    cpi->refresh_last_frame = !encode_show_existing_frame(cm);
     cpi->refresh_golden_frame = 0;
     cpi->refresh_bwd_ref_frame = 0;
     cpi->refresh_alt2_ref_frame = 0;
@@ -3684,7 +3684,7 @@
     shift_last_ref_frames(cpi);
     cpi->ref_fb_idx[0] = tmp;
 
-    assert(encode_show_existing_frame(cm) == 0);
+    assert(!encode_show_existing_frame(cm));
     memcpy(cpi->interp_filter_selected[LAST_FRAME],
            cpi->interp_filter_selected[0],
            sizeof(cpi->interp_filter_selected[0]));