Do not unregister listeners on unexpected data

[Hawk777]

If a registered event listener callback returns false, the listener is unregistered. A network packet arriving on a different port, or a message with a missing part, should not cause the syslog system to shut down. Return nil instead of false if those sanity checks fail, to keep the listener registered.

[XeonSquared]

Oops, didn't know about that little detail. Will have to investigate other services.