checker:event_handler fails on a "remove" event

[timmysilv]

When the self.targets table is populated, the code uses hostname or ip in case the user did not provide a hostname. However, when the same table is accessed for a "remove" event, the code simply refers to target_found.hostname, which results in an attempt to access a nil key.

Adding: or target_found.ip to that key should suffice.