The issue was that the IK constraint wasn't set up correctly,
in such a way that the bone that had the constraint had no parents.
(refer to the file in the bugreport)
This lead to a crash when trying to get the tip of the chain because
`pchan_tip->parent` was a `nullptr`.
Fix it by only getting the parent if it's not a `nullptr`.
This has the side effect of not being able to move the bone in the test file.
Caused by: 0c2afa7c17
Pull Request: https://projects.blender.org/blender/blender/pulls/115788
f489b84316