Fix FTL crashes in ARP table parsing

[DL6ER]

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide.
• I have checked that another pull request for this purpose does not exist.
• I have considered, and confirmed that this submission will be valuable to others.
• I accept that this submission may not be used, and the pull request closed at the will of the maintainer.
• I give this submission freely, and claim no ownership to its content.

How familiar are you with the codebase?:

10

---

This PR fixes a bug in release/v5.0 reported in #718.

The underlying heap memory access violation occurred only when there are both, multiple addresses for the same devices in the ARP table, and only some of these addresses are already known to FTL (i.e., they sent DNS queries to the Pi-hole before).
If either all addresses of a device (or likely none) were already known to FTL because they made requests in the past, this bug doesn’t show up.
This is likely because users with IPv6 are much more likely to see this bug as there is a high likelihood that an interface has at least one address that has never been used to ask Pi-hole any questions. Multiple addresses on the same interface are rather seldom in the IPv4-world.

As a side-effect of my code review, I also found two small memory leaks which are now fixed. There is a small remaining leak in the system-provided function gethostbyaddr() but we cannot do anything about it. The worst seems to be leaking 110 bytes once during the first name resolution. This sounds perfectly acceptable.