KAZOO-3320 reject subscribe requests if there is nothing to subscribe to (or from)

kamailio/presence-role.cfg

@@ -48,6 +48,20 @@ route[HANDLE_SUBSCRIBE]
             exit;
         }
 
+        if ($tU == $null) {
+            xlog("L_INFO", "$ci|stop|ignoring subscribe with empty TO username from a $ua");
+            sl_send_reply(400, "Missing TO username");
+            t_release();
+            exit;
+        }
+
+        if ($fU == $null) {
+            xlog("L_INFO", "$ci|stop|ignoring subscribe with empty FROM username from a $ua");
+            sl_send_reply(400, "Missing FROM username");
+            t_release();
+            exit;
+        }
+
         handle_subscribe();
 
         ##RabbitMQ