Making local an intermediate collection in a liboverride hierarchy e.g.
will likely break the link between the objects in that collection, and
their hierarchy root ID (typically the top overridden collection).
This is fixed by this commit, by:
* Adding a new util, `lib_override_root_is_valid`, that check whether a
given liboverride ID is still 'connected' to its hierarchy root.
* Using this test in `BKE_lib_override_library_main_hierarchy_root_ensure`
to strengthen the detection of invalid root hierarchy pointers.
699d071414