Don't access already freed memory in IRC_KILL()

Don't access already freed memory in IRC_KILL()

It is not possible to call Conn_Close() after Client_Destroy() has been
called, because Conn_Close wants to access the CLIENT structure which
then has been freed already.

Fix IRC_KILL to use Conn_Close() for local clients and Client_Destroy()
for remote clients only (and never both).