Logout hook fires even when there's no nodejs data

I think on sites where nodejs isn't loaded everywhere, you can get into situations where the nodejs session key isn't always set, so when a user logs out, there's no reason (or even way) to log them out of the nodejs server. This is the error I've seen:

Notice: Undefined index: nodejs_config in nodejs_user_logout() (line 520 of /sites/all/modules/nodejs/nodejs.module).

Attached patch should fix this.