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 lets you block connections to any remote servers not on a
whitelist.
``` {.lua}
modules_enabled = {
-- other modules --
"s2s_whitelist",
}
s2s_whitelist = {
"example.org",
}
```