zebra: don't deref vxlan-vni array (backport #15647)

[mergify]

Null-check the vxlan-vni array info coming into zebra from the dataplane.

Fixes: #15564

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This is an automatic backport of pull request #15647 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 75ef259 has failed:

On branch mergify/bp/stable/8.5/pr-15647
Your branch is up to date with 'origin/stable/8.5'.

You are currently cherry-picking commit 75ef259b1.
  (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)

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

no changes added to commit (use "git add" and/or "git commit -a")

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

[mjstapp]

I think the bug was introduced with interface dataplane changes in 9.1 - not present in 8.5.