Fix CVE-2016-7568 (integer overflow in gdImageWebpCtx()):

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-7568

Patch copied from upstream source repository:

https://github.com/libgd/libgd/commit/2806adfdc27a94d333199345394d7c302952b95f

From 2806adfdc27a94d333199345394d7c302952b95f Mon Sep 17 00:00:00 2001
From: trylab <trylab@users.noreply.github.com>
Date: Tue, 6 Sep 2016 18:35:32 +0800
Subject: [PATCH] Fix integer overflow in gdImageWebpCtx

Integer overflow can be happened in expression gdImageSX(im) * 4 *
gdImageSY(im). It could lead to heap buffer overflow in the following
code. This issue has been reported to the PHP Bug Tracking System. The
proof-of-concept file will be supplied some days later. This issue was
discovered by Ke Liu of Tencent's Xuanwu LAB.
---
 src/gd_webp.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/gd_webp.c b/src/gd_webp.c
index 8eb4dee..9886399 100644
--- a/src/gd_webp.c
+++ b/src/gd_webp.c
@@ -199,6 +199,14 @@ BGD_DECLARE(void) gdImageWebpCtx (gdImagePtr im, gdIOCtx * outfile, int quality)
 		quality = 80;
 	}
 
+	if (overflow2(gdImageSX(im), 4)) {
+		return;
+	}
+
+	if (overflow2(gdImageSX(im) * 4, gdImageSY(im))) {
+		return;
+	}
+
 	argb = (uint8_t *)gdMalloc(gdImageSX(im) * 4 * gdImageSY(im));
 	if (!argb) {
 		return;
-- 
2.10.0

