mod_c2s_limit_sessions: Fix check when it's the first connection Creation of the sessions[username] entry happens after the "pre-resource-bind" event, before the "resource-bind" event. Thus, for a users first connection, it may not exist. This led to an 'attempt to index a nil value' error, fixed here.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```