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 remote servers at the s2s
level.
``` {.lua}
modules_enabled = {
-- other modules --
"s2s_blacklist",
}
s2s_blacklist = {
"proxy.eu.jabber.org",
}
```