GUAC-1427: Do not invoke createTunnel() within WebSocket tunnels unless close() is guaranteed to eventually run.

guacamole-common/src/main/java/org/glyptodon/guacamole/websocket/GuacamoleWebSocketTunnelEndpoint.java

@@ -207,6 +207,10 @@ public abstract class GuacamoleWebSocketTunnelEndpoint extends Endpoint {
     @OnMessage
     public void onMessage(String message) {
 
+        // Ignore inbound messages if there is no associated tunnel
+        if (tunnel == null)
+            return;
+
         GuacamoleWriter writer = tunnel.acquireWriter();
 
         try {