mod_anti_spam: Gracefully handle failure to parse CIDR in IP RTBL This should prevent a traceback in case the IP address in the RTBL cannot be parsed (either due to a malformed address being published, or due to a bug in our parsing code).

This module writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.