AVLTreeBase: Clear node's pointers after removal.

To be safe, in case something tries to double-remove.

src/system/kernel/util/AVLTreeBase.cpp

@@ -576,6 +576,7 @@ AVLTreeBase::_Remove(AVLTreeNode* node)
 		*nodeP = NULL;
 	}
 
+	node->parent = node->left = node->right = NULL;
 	fNodeCount--;
 
 	// do the balancing