use send_reply instead of sl_send_reply

* send_reply checks if there's a ongoing transaction and will call proper function to send the reply

(cherry picked from commit ff12d456b5)

kamailio/default.cfg

@@ -487,7 +487,7 @@ route[PREPARE_INITIAL_REQUESTS]
         if (t_check_trans()) {
             route(RELAY);
         } else {
-            sl_send_reply("481", "Call leg/transaction does not exist");
+            send_reply("481", "Call leg/transaction does not exist");
         }
         exit();
     } else if (is_method("ACK")) {