Fixed compilation error (bug 340765) reported by dmacks with patch

* relaxng.c: Fixed compilation error (bug 340765) reported by
  dmacks with patch supplied by Graham Bennett.
diff --git a/relaxng.c b/relaxng.c
index 478f31a..dd8b6cd 100644
--- a/relaxng.c
+++ b/relaxng.c
@@ -7394,7 +7394,7 @@
     if (root == NULL) {
         xmlRngPErr(ctxt, (xmlNodePtr) doc,
 	           XML_RNGP_EMPTY, "xmlRelaxNGParse: %s is empty\n",
-                   (ctxt->URL ? ctxt->URL : "schemas"), NULL);
+                   (ctxt->URL ? ctxt->URL : BAD_CAST "schemas"), NULL);
 	
         xmlFreeDoc(ctxt->document);
         ctxt->document = NULL;