d174959759
The issue was introduced by b35831ad6c.
Since that commit `tree->boundary` will always be non-nullptr, even
when the target mesh had no boundaries. Some code was still relying
on the fact that `tree->boundary != nullptr` means the mesh has
boundary.
Update shrinkwrap code for this fact, avoiding access past array
boundaries.
Pull Request: https://projects.blender.org/blender/blender/pulls/120054