Fix mod_muc_max_occupants to properly ignore whitelisted users

In a typical Jitsi Meet setup, this plugin can be used to limit the number of
occupants in a meeting room, while ignoring "utility" users. Such a
configuration could be:

    muc_max_occupants = 2
    muc_access_whitelist = {
        "focus@auth.meet.jitsi";
    }

It would be expected that this configuration allows two users to attend the
meeting room, but in practice only one is allowed, because the whitelist is not
honoured.

This commit fixes it by actually updating the `user` and `domain` variables
being checked. After this change, the scenario above works just fine.

resources/prosody-plugins/mod_muc_max_occupants.lua

 		-- For each person in the room that's not on the whitelist, subtract one
 		-- from the count.
 		for _, occupant in room:each_occupant() do
+			user, domain, res = split_jid(occupant.bare_jid);
 			if not whitelist:contains(domain) and not whitelist:contains(user..'@'..domain) then
 				slots = slots - 1
 			end