Display an error message if the path_info doesn't contain the required elements

(sub-code and filename).

web/root/msgs/attachments.ssjs

@@ -17,6 +17,10 @@ var sub=path[1];
 var id=parseInt(path[2]);
 var filename=path[3];
 
+if(sub==undefined || filename==undefined) {
+    error("Invalid path: " + http_request.path_info);
+}
+
 var msgbase = new MsgBase(sub);
 if(msgbase.open!=undefined && msgbase.open()==false) {
 	error(msgbase.last_error);