security: fix remotely triggerable crash in SSL/TLS code

When a server is running with SSL/TLS support compiled in,
it is trivial to crash the server by sending an MOTD request
via another server in the network.

- ONLY servers without ssl/tls support compiled in are not affected.
  Disabling SSL in the configuration (no ssl listening ports, etc)
  does NOT help.
- servers that are running standalone (i.e., not connected to any
  other servers) are not affected, either.

This affects all ngircd releases since ngircd 13 (earlier versions
have no SSL/TLS support).