Fixed memory leak when authentication is wrong

[xorduna]

We had an mqtt client that was continously attempting to login with a wrong token into our broker and the memory usage was going up and up.

After digging into the problem, we found that after the connection was closed, two goroutines were still running: writeLoop() and serve(). serve() was blocked waiting for the closing of the client.wg.Wait(), and this wait group was still waiting for the writeLoop() to end. And the writeLoop() was blocked because it was waiting for the client.close channel. The defer in the serve() closed client.closed, but not client.close.

On this date, we still don't see the difference between client.closed and client.close, but closing client.close in case there is an error was necessary to end both go routines.

In any case, I am open to the discussion.

Thanks