don't pass undefined port to ns_bucket:is_port_free

Change-Id: I453668fcac8951fe4dd66fb1771ac987e44a9741
Reviewed-on: http://review.couchbase.org/40447
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>