Fix CVE-2016-8670 (buffer overflow in dynamicGetbuf()):

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-8670
http://seclists.org/oss-sec/2016/q4/138

Patch copied from upstream source repository:

https://github.com/libgd/libgd/commit/53110871935244816bbb9d131da0bccff734bfe9

From 53110871935244816bbb9d131da0bccff734bfe9 Mon Sep 17 00:00:00 2001
From: "Christoph M. Becker" <cmbecker69@gmx.de>
Date: Wed, 12 Oct 2016 11:15:32 +0200
Subject: [PATCH] Avoid potentially dangerous signed to unsigned conversion

We make sure to never pass a negative `rlen` as size to memcpy(). See
also <https://bugs.php.net/bug.php?id=73280>.

Patch provided by Emmanuel Law.
---
 src/gd_io_dp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/gd_io_dp.c b/src/gd_io_dp.c
index 135eda3..228bfa5 100644
--- a/src/gd_io_dp.c
+++ b/src/gd_io_dp.c
@@ -276,7 +276,7 @@ static int dynamicGetbuf(gdIOCtxPtr ctx, void *buf, int len)
 	if(remain >= len) {
 		rlen = len;
 	} else {
-		if(remain == 0) {
+		if(remain <= 0) {
 			/* 2.0.34: EOF is incorrect. We use 0 for
 			 * errors and EOF, just like fileGetbuf,
 			 * which is a simple fread() wrapper.
-- 
2.10.1