fix a bug in SetQuitMessage
fix a bug in SetQuitMessage

Quit messages aren't supposed to clobber each other under any
circumstances, but actually only the default "Connection closed"
message was non-clobbering, all others would clobber.

DanielOaks/oragono,DanielOaks/oragono,oragono/oragono,oragono/oragono,oragono/oragono