Address VS2010 compile warning

xmlparse.c(1893): warning C4244: 'return' : conversion from '__int64' to 'XML_Index', possible loss of data

Reported by tbeu <tc@tbeu.de>
diff --git a/expat/lib/xmlparse.c b/expat/lib/xmlparse.c
index 0cce2ed..b308e67 100644
--- a/expat/lib/xmlparse.c
+++ b/expat/lib/xmlparse.c
@@ -1890,7 +1890,7 @@
 XML_GetCurrentByteIndex(XML_Parser parser)
 {
   if (eventPtr)
-    return parseEndByteIndex - (parseEndPtr - eventPtr);
+    return (XML_Index)(parseEndByteIndex - (parseEndPtr - eventPtr));
   return -1;
 }