Fix access violation exception on shutdown (#1977)

When nulling the GC handles on shutdown the reference count of all objects pointed to by the IntPtr in the `CLRObject.reflectedObjects` are zero.
This caused an exception in some scenarios because `Runtime.PyObject_TYPE(reflectedClrObject)` is called while the reference counter is at zero.

After `TypeManager.RemoveTypes();` is called in the `Runtime.Shutdown()` method, reference count decrements to zero do not invoke `ClassBase.tp_clear` for managed objects anymore which normally is responsible for removing references from `CLRObject.reflectedObjects`. Collecting objects referenced in `CLRObject.reflectedObjects` only after leads to an unstable state in which the reference count for these object addresses is zero while still maintaining them to be used for further pseudo-cleanup. In that time, the memory could have been reclaimed already which leads to the exception.