Prevent a possible segmentation fault (#4141) (backport #4147)

[mergify]

This is an automatic backport of pull request #4147 done by Mergify.
Cherry-pick of 654e7e0 has failed:

On branch mergify/bp/humble/pr-4147
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 654e7e0d.
  (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:   nav2_smac_planner/src/analytic_expansion.cpp

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

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.com

[mergify]

Cherry-pick of 654e7e0 has failed:

On branch mergify/bp/humble/pr-4147
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 654e7e0d.
  (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:   nav2_smac_planner/src/analytic_expansion.cpp

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

[mergify]

@mergify[bot], all pull requests must be targeted towards the main development branch.
Once merged into main, it is possible to backport to @humble, but it must be in main
to have these changes reflected into new distributions.