guarding against Errors on Socket Disconnect

Saw this 

```
TypeError: Cannot call method 'indexOf' of undefined
    at Socket.<anonymous> (/home/admin/NodeBB/src/websockets.js:108:33)
    at Socket.EventEmitter.emit [as $emit] (events.js:95:17)
    at Socket.onDisconnect (/home/admin/NodeBB/node_modules/socket.io/lib/socket.js:153:10)
    at SocketNamespace.handleDisconnect (/home/admin/NodeBB/node_modules/socket.io/lib/namespace.js:229:46)
```

and this

```
/home/admin/NodeBB/src/websockets.js:113
			if (userSockets[uid].length === 0) {
			                    ^
TypeError: Cannot read property 'length' of undefined
    at Socket.<anonymous> (/home/admin/NodeBB/src/websockets.js:113:24)
```

in my logs, when users are disconnecting
Merge pull request #704 from akhoury/master

guarding against Errors on Socket Disconnect