Implement glooxwrapper::MUCRoom argument for muc event handlers.

I.e. don't dereference NULL if the user of the glooxwrapper library reads from the provided room argument.
A step towards being able to use XmppClient with multiple rooms.

Differential Revision: https://code.wildfiregames.com/D2287
Tested on: clang 8.0.1, Jenkins