Follow-up upstream fix for CVE-2015-1283 to not rely on undefined
behavior.

Adapted from a patch from Debian (found in Debian package version
2.1.0-6+deb8u2) to apply to upstream code:

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

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

diff --git a/lib/xmlparse.c b/lib/xmlparse.c
index 0f6f4cd..5c70c17 100644
--- a/lib/xmlparse.c
+++ b/lib/xmlparse.c
@@ -1727,7 +1727,8 @@ 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));
     if (neededSize < 0) {
       errorCode = XML_ERROR_NO_MEMORY;
       return NULL;
@@ -1759,7 +1760,8 @@ 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);
       } while (bufferSize < neededSize && bufferSize > 0);
       if (bufferSize <= 0) {
         errorCode = XML_ERROR_NO_MEMORY;
-- 
2.8.3