Fix edge-case interpretation of DHCP lease expiration field

[DL6ER]

What does this implement/fix?

Fix interpretation of lease expiration field in case it is 0. This does not mean 1st of January 1970 but really a never expiring lease:

Related issue or feature (if applicable): https://discourse.pi-hole.net/t/lease-expired-54-years-ago/67888

Pull request in docs with documentation (if applicable): N/A

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