expat: patch CVE-2015-1283 and CVE-2016-0718 (close #15561)

Note, CVE-2015-1283 is already patched in expat version 2.1.1 but, as
explained in the patch, the fix was insufficient.

pkgs/development/libraries/expat/CVE-2015-1283-refix.patch

@@ -0,0 +1,37 @@
+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. It
+ does not really work: https://godbolt.org/g/Zl8gdF
+
+---
+ 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
+@@ -1693,7 +1693,8 @@ 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;
+@@ -1725,7 +1726,8 @@ 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;
+-- 
+2.8.2
+