mod_sasl2_fast: Fix traceback when no cb_data is available (thanks riau) We already verify (earlier in the process) that cb_data is present if we are using a mechanism that requires it.

This module blocks configured queries that change server state.

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

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