Fix rev-server crash

[DL6ER]

What does this implement/fix?

Fix crash caused by double free() corruption encountered with rev-server addresses with prefix lengths != {8,16,24,32}.

The patch submitted upstream is tracked here: pi-hole/dnsmasq#15

We decide to merge this change now as nothing at all has happened on the mailing list for months by now and users came and asked for the status of our bug fix.

---

Related issue or feature (if applicable): https://discourse.pi-hole.net/t/todays-update-kills-pihole-v6/70739

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

---

By submitting this pull request, I confirm the following:

1. 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.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

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

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/todays-update-kills-pihole-v6/70739/14