Fix crash for invalid index in force torque GUI plugin (backport #3601)

[mergify]

🦟 Bug fix

Fixes crash when selecting models and the ApplyForceTorque plugin is loaded.

Summary

Originally found by @iche033 in intrinsic-dev/aic#553, it seems that due to a race condition sometimes the linkIndex is set to an uninitialized value (-1). We then try to do an array access with that index and crash.
Just added a boundary check so we don't do that until the next call to the Update function.

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the fix (as needed)
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
• While waiting for a review on your PR, please help review another open pull request to support the maintainers
• Was GenAI used to generate this PR? If so, make sure to add "Generated-by" to your commits. (See this policy for more info.)

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by and Generated-by messages.

---

This is an automatic backport of pull request #3601 done by Mergify.

[mergify]

Cherry-pick of de5e2a2 has failed:

On branch mergify/bp/ign-gazebo6/pr-3601
Your branch is up to date with 'origin/ign-gazebo6'.

You are currently cherry-picking commit de5e2a21.
  (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/rm <file>..." as appropriate to mark resolution)
	deleted by us:   src/gui/plugins/apply_force_torque/ApplyForceTorque.cc

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