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.
Introduction
============
This module creates stub configuration files for newly activated hosts.
Configuration
=============
A single option exists, `persisthosts_path`, which is the path where new
stub configuration files are created. It defaults to `"conf.d"`, and is
treated as relative to the configuration directiory [^1] unless set to
an absolute path.
[^1]: usually `/etc/prosody` on \*nix systems