Make `action_channelbag_ensure()` and `action_fcurve_ensure()` return a
reference. Earlier it returned pointer that should never be `nullptr`,
but in certain cases (which would indicate a bug, as the function's
preconditions aren't met) could technically still be a `nullptr`.
Instead, the preconditions are checked with `BLI_assert()`, and a
release build will simply assume that they are met. This simplifies the
code of the functions themselves, as well as the callers.
For reference: this was discussed in !134866.
No functional changes.
Pull Request: https://projects.blender.org/blender/blender/pulls/134918
6225c3e667