The length modifier was not handling 2 point curves correctly.
This change does two things:
1) Fix the crash by copying the original indices for 2 point curves into `dst_to_src_point`.
2) Fallback to `extend_curves_straight` for extending 2 point curves.
Pull Request: https://projects.blender.org/blender/blender/pulls/130122
43fde8c39c