Invalidate depsgraph.object_instances when freed, this resolves a crash
when accessing the object instances after iteration has finished.
Unlike most other collections, object_instances is only valid while the
iterator is in-memory.
The Python/RNA API needs to inline int/string collection lookups so the
Python instance can be created before the iterator ends.
Reviewed By: mont29, sergey
Ref D15755
21ea499558