gnu: pixman: Work around pointer arithmetic bug.

* gnu/packages/patches/pixman-pointer-arithmetic.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/xdisorg.scm (pixman)[source]: Use it.

gnu-system.am

@@ -586,6 +586,7 @@ dist_patch_DATA =						\
   gnu/packages/patches/perl-tk-x11-discover.patch		\
   gnu/packages/patches/pidgin-add-search-path.patch		\
   gnu/packages/patches/pingus-sdl-libs-config.patch		\
+  gnu/packages/patches/pixman-pointer-arithmetic.patch		\
   gnu/packages/patches/plotutils-libpng-jmpbuf.patch		\
   gnu/packages/patches/polkit-drop-test.patch			\
   gnu/packages/patches/portaudio-audacity-compat.patch		\

gnu/packages/patches/pixman-pointer-arithmetic.patch

@@ -0,0 +1,15 @@
+Fix <https://bugs.freedesktop.org/show_bug.cgi?id=92027> whereby
+an arithemitic overflow could occur while doing pointer arithmetic,
+leading pixman to use an invalid address as the destination buffer.
+
+--- pixman-0.32.6/pixman/pixman-general.c	2015-09-21 15:14:34.695981325 +0200
++++ pixman-0.32.6/pixman/pixman-general.c	2015-09-21 15:19:48.898355548 +0200
+@@ -144,8 +144,7 @@ general_composite_rect  (pixman_implemen
+     mask_buffer = ALIGN (src_buffer + width * Bpp);
+     dest_buffer = ALIGN (mask_buffer + width * Bpp);
+ 
+-    if (ALIGN (dest_buffer + width * Bpp) >
+-	    scanline_buffer + sizeof (stack_scanline_buffer))
++    if ((width + 1) * Bpp * 3 > sizeof (stack_scanline_buffer))
+     {
+ 	scanline_buffer = pixman_malloc_ab_plus_c (width, Bpp * 3, 32 * 3);

gnu/packages/xdisorg.scm

@@ -150,7 +150,8 @@ following the mouse.")
                ".tar.gz"))
         (sha256
           (base32
-           "0129g4zdrw5hif5783li7rzcr4vpbc2cfia91azxmsk0h0xx3zix"))))
+           "0129g4zdrw5hif5783li7rzcr4vpbc2cfia91azxmsk0h0xx3zix"))
+        (patches (list (search-patch "pixman-pointer-arithmetic.patch")))))
     (build-system gnu-build-system)
     (inputs
       `(("libpng" ,libpng)