gnu: libvpx: Add upstream security fix.

* gnu/packages/patches/libvpx-use-after-free-in-postproc.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/video.scm (libvpx)[source]: Add patch.

gnu/packages/patches/libvpx-use-after-free-in-postproc.patch

@@ -0,0 +1,34 @@
+From 52add5896661d186dec284ed646a4b33b607d2c7 Mon Sep 17 00:00:00 2001
+From: Jerome Jiang <jianj@google.com>
+Date: Wed, 23 May 2018 15:43:00 -0700
+Subject: [PATCH] VP8: Fix use-after-free in postproc.
+
+The pointer in vp8 postproc refers to show_frame_mi which is only
+updated on show frame. However, when there is a no-show frame which also
+changes the size (thus new frame buffers allocated), show_frame_mi is
+not updated with new frame buffer memory.
+
+Change the pointer in postproc to mi which is always updated.
+
+Bug: 842265
+Change-Id: I33874f2112b39f74562cba528432b5f239e6a7bd
+---
+ vp8/common/postproc.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/vp8/common/postproc.c b/vp8/common/postproc.c
+index d67ee8a57..8c292d616 100644
+--- a/vp8/common/postproc.c
++++ b/vp8/common/postproc.c
+@@ -65,7 +65,7 @@ void vp8_deblock(VP8_COMMON *cm, YV12_BUFFER_CONFIG *source,
+   double level = 6.0e-05 * q * q * q - .0067 * q * q + .306 * q + .0065;
+   int ppl = (int)(level + .5);
+ 
+-  const MODE_INFO *mode_info_context = cm->show_frame_mi;
++  const MODE_INFO *mode_info_context = cm->mi;
+   int mbr, mbc;
+ 
+   /* The pixel thresholds are adjusted according to if or not the macroblock
+-- 
+2.19.0
+