Blender Git Loki

LibOverride: Fix (unreported) assert when creating overrides of linked overrides.

We do not want to copy exiting overrides data from linked ID when
creating its local override (be it either a template, or because linked
ID is itself an override of another lib data).

Note that this was not a very serious issue, would just cause some memory
leak since override data is re-created on newly copied local data
anyway.

These use cases have been very little tested so far, but both complex
production pipeline and future restrictive workflow will make them fairly
common...