Blender Git Loki

PyAPI: correct garbage collection for StructRNA types

By default objects are no longer GC tracked, this removes some overhead
although it's not significant in my own testing.

- Only enable GC for StructRNA when 'WITH_PYTHON_SAFETY' is on.
- Only track StructRNA when their 'reference' is set.
- Add missing NULL check when 'WITH_PYTHON_SAFETY' is on
and objects new objects be created.