Update previous fix for CVE-2015-1283 to not rely on undefined behavior.

Copied from Debian, as found in Debian package version 2.1.0-6+deb8u2.

https://sources.debian.net/src/expat/2.1.0-6%2Bdeb8u2/debian/patches/CVE-2015-1283-refix.patch/

From 29a11774d8ebbafe8418b4a5ffb4cc1160b194a1 Mon Sep 17 00:00:00 2001
From: Pascal Cuoq <cuoq@trust-in-soft.com>
Date: Sun, 15 May 2016 09:05:46 +0200
Subject: [PATCH] Avoid relying on undefined behavior in CVE-2015-1283 fix.

---
 expat/lib/xmlparse.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/lib/xmlparse.c b/lib/xmlparse.c
index 13e080d..cdb12ef 100644
--- a/lib/xmlparse.c
+++ b/lib/xmlparse.c
@@ -1695,7 +1695,8 @@ XML_GetBuffer(XML_Parser parser, int len
   }
 
   if (len > bufferLim - bufferEnd) {
-    int neededSize = len + (int)(bufferEnd - bufferPtr);
+    /* Do not invoke signed arithmetic overflow: */
+    int neededSize = (int) ((unsigned)len + (unsigned)(bufferEnd - bufferPtr));
 /* BEGIN MOZILLA CHANGE (sanity check neededSize) */
     if (neededSize < 0) {
       errorCode = XML_ERROR_NO_MEMORY;
@@ -1729,7 +1730,8 @@ XML_GetBuffer(XML_Parser parser, int len
       if (bufferSize == 0)
         bufferSize = INIT_BUFFER_SIZE;
       do {
-        bufferSize *= 2;
+        /* Do not invoke signed arithmetic overflow: */
+        bufferSize = (int) (2U * (unsigned) bufferSize);
 /* BEGIN MOZILLA CHANGE (prevent infinite loop on overflow) */
       } while (bufferSize < neededSize && bufferSize > 0);
 /* END MOZILLA CHANGE */
-- 
2.8.2