fixed disconnect null pointer

src/main/java/com/corundumstudio/socketio/handler/ClientHead.java

@@ -205,7 +205,9 @@ public class ClientHead {
 
     public void disconnect() {
         ChannelFuture future = send(new Packet(PacketType.DISCONNECT));
+		if(future != null) {
 			future.addListener(ChannelFutureListener.CLOSE);
+		}
 
         onChannelDisconnect();
     }