Fix CVE-2017-5896:

https://bugs.ghostscript.com/show_bug.cgi?id=697515
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-5896
http://www.openwall.com/lists/oss-security/2017/02/10/1
https://security-tracker.debian.org/tracker/CVE-2017-5896
https://blogs.gentoo.org/ago/2017/02/09/mupdf-use-after-free-in-fz_subsample_pixmap-pixmap-c/

Patch lifted from upstream source repository:

http://git.ghostscript.com/?p=mupdf.git;h=2c4e5867ee699b1081527bc6c6ea0e99a35a5c27

From 2c4e5867ee699b1081527bc6c6ea0e99a35a5c27 Mon Sep 17 00:00:00 2001
From: Robin Watts <Robin.Watts@artifex.com>
Date: Thu, 9 Feb 2017 07:12:16 -0800
Subject: [PATCH] bug 697515: Fix out of bounds read in fz_subsample_pixmap

Pointer arithmetic for final special case was going wrong.
---
 source/fitz/pixmap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/source/fitz/pixmap.c b/source/fitz/pixmap.c
index a8317127..f1291dc2 100644
--- a/source/fitz/pixmap.c
+++ b/source/fitz/pixmap.c
@@ -1104,6 +1104,7 @@ fz_subsample_pixmap_ARM(unsigned char *ptr, int w, int h, int f, int factor,
 	"@STACK:r1,<9>,factor,n,fwd,back,back2,fwd2,divX,back4,fwd4,fwd3,divY,back5,divXY\n"
 	"ldr	r4, [r13,#4*22]		@ r4 = divXY			\n"
 	"ldr	r5, [r13,#4*11]		@ for (nn = n; nn > 0; n--) {	\n"
+	"ldr	r8, [r13,#4*17]		@ r8 = back4			\n"
 	"18:				@				\n"
 	"mov	r14,#0			@ r14= v = 0			\n"
 	"sub	r5, r5, r1, LSL #8	@ for (xx = x; xx > 0; x--) {	\n"
@@ -1120,7 +1121,7 @@ fz_subsample_pixmap_ARM(unsigned char *ptr, int w, int h, int f, int factor,
 	"mul	r14,r4, r14		@ r14= v *= divX		\n"
 	"mov	r14,r14,LSR #16		@ r14= v >>= 16			\n"
 	"strb	r14,[r9], #1		@ *d++ = r14			\n"
-	"sub	r0, r0, r8		@ s -= back2			\n"
+	"sub	r0, r0, r8		@ s -= back4			\n"
 	"subs	r5, r5, #1		@ n--				\n"
 	"bgt	18b			@ }				\n"
 	"21:				@				\n"
@@ -1249,6 +1250,7 @@ fz_subsample_pixmap(fz_context *ctx, fz_pixmap *tile, int factor)
 		x += f;
 		if (x > 0)
 		{
+			int back4 = x * n - 1;
 			div = x * y;
 			for (nn = n; nn > 0; nn--)
 			{
@@ -1263,7 +1265,7 @@ fz_subsample_pixmap(fz_context *ctx, fz_pixmap *tile, int factor)
 					s -= back5;
 				}
 				*d++ = v / div;
-				s -= back2;
+				s -= back4;
 			}
 		}
 	}
-- 
2.12.0