`SinglyLinkedList::remove` dereferences a null link

In versions before 0.2.1, `SinglyLinkedList::remove` is safe and walks the intrusive list with an unchecked dereference. On an empty list, or when `node` is not in the list, `(*current_elm).next` reads a null pointer. That is undefined behavior. The list head is a raw `*mut Node<T>`, and safe code can construct the empty list.

The maintainer fixed this in 0.2.1 by rejecting those two cases with an unconditional `assert!` before the pointer is followed, matching the upstream Zig `unwrap` on the same paths. 0.2.0 was yanked. Versions 0.1.0 through 0.1.13 are still published and still contain the unchecked walk.