libvpx: Pull from upstream

source/libvpx/vp9/common/vp9_frame_buffers.c

 /*
  *  Copyright (c) 2014 The WebM project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 58 matching lines @@
   int_fb_list->int_fb[i].in_use = 1;
 
   // Set the frame buffer's private data to point at the internal frame buffer.
   fb->priv = &int_fb_list->int_fb[i];
   return 0;
 }
 
 int vp9_release_frame_buffer(void *cb_priv, vpx_codec_frame_buffer_t *fb) {
   InternalFrameBuffer *const int_fb = (InternalFrameBuffer *)fb->priv;
   (void)cb_priv;
-  int_fb->in_use = 0;
+  if (int_fb)
+    int_fb->in_use = 0;
   return 0;
 }