xerces: add upstream security fix

CVE-2017-12627: dereference of a NULL pointer while processing the path
to the DTD.

xerces 3.2.1 includes this patch. But this version also added
AC_RUN_IFELSE to its configure script, making cross compilation harder.

Switching to cmake is also problematic since the minimum required cmake
version is 3.2.0. The host dependencies check currently allows minimum
cmake version 3.1.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/xerces/0001-fix-CVE-2017-12627.patch

@@ -0,0 +1,22 @@
+XMLString: Don't call catString if relativePath is null
+
+https://xerces.apache.org/xerces-c/secadv/CVE-2017-12627.txt
+
+Upstream status: svn revision 1819998
+
+Signed-off-by: Baruch Siach <baruch@tkos.co.il>
+
+--- trunk/src/xercesc/util/PlatformUtils.cpp	2018/01/03 18:58:30	1819997
++++ trunk/src/xercesc/util/PlatformUtils.cpp	2018/01/03 18:59:30	1819998
+@@ -920,7 +920,10 @@
+ 
+     XMLString::subString(tmpBuf, basePath, 0, (basePtr - basePath + 1), manager);
+     tmpBuf[basePtr - basePath + 1] = 0;
+-    XMLString::catString(tmpBuf, relativePath);
++    if (relativePath)
++    {
++        XMLString::catString(tmpBuf, relativePath);
++    }
+ 
+     removeDotSlash(tmpBuf, manager);
+