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);