[NTOS:SE] Minor refactor on NtOpenThreadTokenEx

[NTOS:SE] Minor refactor on NtOpenThreadTokenEx

- Remove a redundant call of ObReferenceObjectByHandle. Not only it didn't make much sense (we reference the object from thread handle and the new thread object referencing the same handle!), specifying a request access of THREAD_ALL_ACCESS for the thread object is kind of suspicious and all of these access rights are unwanted.
- Add some failure checks involving the CopyOnOpen code paths
- Add some DPRINT1 debug prints (concerning the CopyOnOpen code paths as usual)