In some cases, sculpt code currently creates undo steps that are stored
in `step_init` in the undo stack, but then skips actually pushing them.
That can happen when an operator is cancelled (like the transform
operator in #123172) or because pushes in "nested operator calls"
are currently explicitly disabled. That happens when calling the brush
operator from a script.
It turns out the undo code never freed the `step_init`, probably because
it assumed it would be pushed to be part of the stack afterwards.
Personally I'm not convinced that separating undo step creation into
two stages with `step_encode_init` and `step_encode` is a great design
or a necessary one, but I'm trying not to get into that deeper right now.
Fixes#123172
Pull Request: https://projects.blender.org/blender/blender/pulls/123331
1b80125a67