Trap request to display non-existant frame

If show_existing_frame indicates that the decoder should
display an existing (previously decoded) frame, add a
check to make sure that the signaled buffer does contain
a valid decoded frame.

Change-Id: Iac8c686b321827414d69a3f2d0467566911bcba2

vp9/decoder/vp9_decodeframe.c

diff --git a/vp9/decoder/vp9_decodeframe.c b/vp9/decoder/vp9_decodeframe.c
index 791d0f2..4f3aca1 100644
--- a/vp9/decoder/vp9_decodeframe.c
+++ b/vp9/decoder/vp9_decodeframe.c
@@ -1120,6 +1120,12 @@
   if (cm->show_existing_frame) {
     // Show an existing frame directly.
     const int frame_to_show = cm->ref_frame_map[vp9_rb_read_literal(rb, 3)];
+
+    if (cm->frame_bufs[frame_to_show].ref_count < 1)
+      vpx_internal_error(&cm->error, VPX_CODEC_UNSUP_BITSTREAM,
+                         "Buffer %d does not contain a decoded frame",
+                         frame_to_show);
+
     ref_cnt_fb(cm->frame_bufs, &cm->new_fb_idx, frame_to_show);
     pbi->refresh_frame_flags = 0;
     cm->lf.filter_level = 0;