mod_sasl2_fast: Improve handling when SASL profile unexpectedly lacks CB This fixes a traceback reported by riau, but likely does not solve the underlying cause, whatever that is.

This module blocks configured queries that change server state.

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

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