Prevent unwanted external entity reference

For https://bugzilla.gnome.org/show_bug.cgi?id=780691

* parser.c: add a specific check to avoid PE reference

parser.c

@@ -8123,6 +8123,15 @@ xmlParsePEReference(xmlParserCtxtPtr ctxt)
 	    if (xmlPushInput(ctxt, input) < 0)
 		return;
 	} else {
+	    if ((entity->etype == XML_EXTERNAL_PARAMETER_ENTITY) &&
+	        ((ctxt->options & XML_PARSE_NOENT) == 0) &&
+		((ctxt->options & XML_PARSE_DTDVALID) == 0) &&
+		((ctxt->options & XML_PARSE_DTDLOAD) == 0) &&
+		((ctxt->options & XML_PARSE_DTDATTR) == 0) &&
+		(ctxt->replaceEntities == 0) &&
+		(ctxt->validate == 0))
+		return;
+
 	    /*
 	     * TODO !!!
 	     * handle the extra spaces added before and after