Blender Git Loki

Fix T80104: Crash on making material local.

Problem is again with the embedded data, we want to make those local
together with their owner ID, but sometimes we are actually dealing with
copies here, which are inheritently already local.

Code did not considered that possibility before, leading to access to a
NULL `lib` pointer.

This should also be back-ported to 2.83 LTS release.

Maniphest Tasks: T80104

Differential Revision: https://developer.blender.org/D8731