Bug 1256558 - Change MUST_CONVERT to avoid C4311 in VS2015; r=khuey

This code is cherry picked from the trunk version of expat.

MozReview-Commit-ID: 8RDaArq2BwO

--HG--
extra : rebase_source : 72d9b56ec2d6d17a99e578017f0e5828ffd3aa1f

parser/expat/lib/xmlparse.c

@@ -26,7 +26,9 @@
 #define XmlGetInternalEncoding XmlGetUtf16InternalEncoding
 #define XmlGetInternalEncodingNS XmlGetUtf16InternalEncodingNS
 #define XmlEncode XmlUtf16Encode
-#define MUST_CONVERT(enc, s) (!(enc)->isUtf16 || (((unsigned long)s) & 1))
+
+/* Using pointer subtraction to convert to integer type. */
+#define MUST_CONVERT(enc, s) (!(enc)->isUtf16 || (((char *)(s) - (char *)NULL) & 1))
 typedef unsigned short ICHAR;
 #else
 #define XML_ENCODE_MAX XML_UTF8_ENCODE_MAX