Don't emit clientDisconnected event when force close client.

Hi, I got a problem. When I force close a client use `CTRL+C` , Mosca not emit `clientDisconnected` event. Here is the code:

* Server
```js
var server = new mosca.Server(options);
server.on('clientDisconnected', function () {
    console.log('client gone');
})
```

* Client
```js
var client = mqtt.createClient();
// Works fine, mosca got the clientDisconnected event.
client.end();
```
```js
var client = mqtt.createClient();
// Use CTRL+C kill the process and mosca not got the clientDisconnected event.
```