iplog: Floored lease time for "tolerance"

[dwlfrth]

When opening an iplog entry (either creating a new one or updating an existing one), PacketFence is using the actual lease time of a given DHCP lease (if provided) or set a NULL (which defaults to 0000-00-00 00:00:00) as the 'end_time' for the record.

It looks like more and more device "may not care" about the provided lease time or it may be some other reason, but the thing is, we are seeing more and more issues where "The device cannot be found in PacketFence database" when trying to reach the captive-portal, which is the result of the portal not being able to get an "open" iplog entry to do the IP <-> MAC lookup.