ripngd: fix data handling in several places (backport #21217)

[mergify]

Don't accept responses unless interface is configured; be more careful with route_node before dereferencing the info pointer; validate min and max packet size before processing.

---

This is an automatic backport of pull request #21217 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 35e3a1e has failed:

On branch mergify/bp/stable/10.4/pr-21217
Your branch is up to date with 'origin/stable/10.4'.

You are currently cherry-picking commit 35e3a1e02.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   ripngd/ripngd.h

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   ripngd/ripngd.c

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[greptile-apps]

Target branch is not in the allowed branches list.

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.