In a cross-realm situation the client KDC exchange may use on orphaned
strengthen_key (from the previous exchange) if the current KDC
doesn't not support FAST and the previous KDC supported it.
Otherwise init_creds_step() or fast_tgs_strengthen_key()
generate the reply key.
BUG: https://bugzilla.samba.org/show_bug.cgi?id=15005
Signed-off-by: Stefan Metzmacher <metze@samba.org>