Protect this accessor with a local static mutex when it needs to
create/write data.
Ideally accessors should never create or modify data, but there are some
cases where this bad behavior is currently unavoidable.
This is the case of the Pointer accessor when the actual IDProperty has
not yet been created.
NOTE: this fixes a memory leak in liboverride diffing process when
several different overrides use a same linked reference ID.
Differential Revision: https://developer.blender.org/D12060
317f09ebf9