mod_easy_invite: backport: ensure session exists before accessing it Backport from https://github.com/snikket-im/snikket-server This prevents a 500 issue when using mod_register_web.

This module blocks configured queries that change server state.

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

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