Fix ArrayIndexOutOfBoundsException in IrcMessage::getHostmask() with senders with no "!"

I don't know why this happens, but my quasselcore appears to be sending occasional senders with a nick/host string without a "!" in. This would cause a crash when displaying buffers with affected senders.

This might be limited to my core or the MySQL support patch that we use, but a try-catch around a far-from-guaranteed function call can't hurt :)