What does this implement/fix?

This PR adds a commit submitted to the dnsmasq-discuss mailing list. It has been verified by users to prevent use-after-free bugs possibly even resulting in a crash when names for meanwhile expired DHCP leases are queried via DNS PTR.

In case this patch is not accepted 1:1 for dnsmasq upstream, we can easily revert this PR later.

Related issue or feature (if applicable): N/A

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

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I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
I have commented my proposed changes within the code.
I am willing to help maintain this change if there are issues with it later.
It is compatible with the EUPL 1.2 license
I have squashed any insignificant commits. (git rebase)

Checklist:

[x] The code change is tested and works locally.
[x] I based my code and PRs against the repositories developmental branch.
[x] I signed off all commits. Pi-hole enforces the DCO for all contributions
[x] I signed all my commits. Pi-hole requires signatures to verify authorship
[x] I have read the above and my PR is ready for review.

pi-hole / FTL

Update DNS records after pruning DHCP leases #1965

What does this implement/fix?

Checklist: