Fix a portability issue on Windows

Apparently an verflow when comparing macro and unsigned long
diff --git a/parser.c b/parser.c
index d61c216..0bf1aa2 100644
--- a/parser.c
+++ b/parser.c
@@ -2039,8 +2039,11 @@
 	xmlGROW (ctxt);
 
 static void xmlGROW (xmlParserCtxtPtr ctxt) {
-    if ((((ctxt->input->end - ctxt->input->cur) > XML_MAX_LOOKUP_LIMIT) ||
-         ((ctxt->input->cur - ctxt->input->base) > XML_MAX_LOOKUP_LIMIT)) &&
+    unsigned long curEnd = ctxt->input->end - ctxt->input->cur;
+    unsigned long curBase = ctxt->input->cur - ctxt->input->base;
+
+    if (((curEnd > (unsigned long) XML_MAX_LOOKUP_LIMIT) ||
+         (curBase > (unsigned long) XML_MAX_LOOKUP_LIMIT)) &&
          ((ctxt->input->buf) && (ctxt->input->buf->readcallback != (xmlInputReadCallback) xmlNop)) &&
         ((ctxt->options & XML_PARSE_HUGE) == 0)) {
         xmlFatalErr(ctxt, XML_ERR_INTERNAL_ERROR, "Huge input lookup");