Fix show existing frame

- Disable mode info update in case where current frame is coded
  as "show existing frame".
- Should fix issue 676.

Change-Id: Ibee681850eb307f982da6528d3e31cb94f881c08
diff --git a/vp9/decoder/vp9_decodeframe.c b/vp9/decoder/vp9_decodeframe.c
index 9b6740e..9d82cac 100644
--- a/vp9/decoder/vp9_decodeframe.c
+++ b/vp9/decoder/vp9_decodeframe.c
@@ -1067,7 +1067,8 @@
   cm->version = vp9_rb_read_bit(rb);
   RESERVED;
 
-  if (vp9_rb_read_bit(rb)) {
+  cm->show_existing_frame = vp9_rb_read_bit(rb);
+  if (cm->show_existing_frame) {
     // show an existing frame directly
     int frame_to_show = cm->ref_frame_map[vp9_rb_read_literal(rb, 3)];
     ref_cnt_fb(cm->fb_idx_ref_cnt, &cm->new_fb_idx, frame_to_show);