gh-135909: Assert incoming refcnt != 0 for the free threaded GC.

[andrewreds]

This is to catch out double deallocation bugs. Likely from a faulty .tp_dealloc allowing the GC to run before the object is untracked.

This assert matches the one added to the GIL build, in 780c497972f367df86cf0f6f3b7c2aecf1e078d6. The call to validate_refcounts was moved up to start of the GC because queue_untracked_obj_decref() creates it own zero reference count garbage.

• Issue: gh-135909

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