Fix a few small mistakes in the action baking code:
- Assigning an action slot should only happen after the action itself has
been assigned.
- `_ensure_channelbag_exists()` didn't actually ensure the channelbag
always exists; now it also creates the layer & strip if necessary.
Pull Request: https://projects.blender.org/blender/blender/pulls/135853
9c1845dbf2