* Simplify and clarify logic in `BKE_undosys_step_undo/redo_with_data_ex`,
by adding early return on invalid situations, renaming some variables,
and adding comments.
* Add more sanity checks in those functions.
No behavioral change are expected here, besides in potential edge-case,
invalid situations.
This is a preliminary change, before some deeper modifications of
`BKE_undosys` undo/redo API.
Differential Revision: https://developer.blender.org/D10033
26fd55fad1