Fix LinkedIterator.remove() for the last live element

[HaHaWTH]

LinkedIterator.findNext() updates curr only when it finds another live node. When next() returns the last live element, curr remains null or points to the previously returned node, causing the iterator to remove the wrong element.

Logs

Exception found on Paper: https://mclo.gs/SlxTMFc

Minimal reproduce code

public void wwwwwwwww() {
    MultiThreadedQueue<String> queue = new MultiThreadedQueue<>(List.of("A"));
    Iterator<String> itr = queue.iterator();
  
    itr.next(); // A
    itr.remove(); // IllegalStateException
}

Fix

Record the current next node before clearing the final iterator state. This maintains exact-node O(1) removal and keeps MTQueue's concurrent semantics.