cluster/websocket: Fix null deref at WebSocket server shutdown

WebSocket clients that connected with the wrong URL do not have
a backend attached. If a dispatcher is terminated while these
clients are still connected, a null deref would happen.

This was found while running all cluster/websocket tests in a loop
for a long time, tickling a segfault during the bad-url test.

src/cluster/websocket/WebSocket.cc

@@ -231,6 +231,8 @@ void WebSocketEventDispatcher::Terminate() {
                  wsc->getRemotePort());
 
         QueueReply(WebSocketCloseReply{wsc, 1001, "Terminating"});
+
+        if ( backend )
             backend->Terminate();
     }