mem-pool: Fix issue with make-before-break reauth and multiple IKE_SAs
If uniqueness checks are disabled and multiple IKE_SAs with the same
identities are created, an offline lease could have gotten reassigned
during a make-before-break reauthentication if such an SA was closed
earlier. Checking for an online lease for the same client (IP/port)
first ensures that the correct IP is reassigned during the
reauthentication.