Fix seg fault in t::geometry::TriangleMesh::SelectByIndex for negative index

[nsaiapova]

Sorry about the issue. It was introduced by #6415. When I moved from forbidding the invalid indices to ignoring them, I forgot to actually ignore them, which led to a possible invalid data access by a negative index.
That went unnoticed, as the test was only in python and was flaky.

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #

Motivation and Context

A data access by negative index.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.
  FYI @ssheorey

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[ssheorey]

Thanks for the quick bugfix!