THMAP_NULL
ASSERT(node_locked_p(node) || node->parent == THMAP_NULL);
ASSERT(atomic_load_relaxed(&node->slots[slot]) == THMAP_NULL);
ASSERT(atomic_load_relaxed(&node->slots[slot]) != THMAP_NULL);
atomic_store_relaxed(&node->slots[slot], THMAP_NULL);
expected = THMAP_NULL;
ASSERT(parent->parent == THMAP_NULL);
atomic_store_relaxed(&thmap->root[rslot], THMAP_NULL);