Null Reference Exception (#817)

When `n.next = tail` is true, we assign `n` to `tail` and `null` to `tail.next`, so `n.next` also becomes `null`. Then we assign `n.next.next` (because now `n.next` is `null`), we try to get `next` of `null`. That is why we should add an `else` case to check if `n.next` is not equal to `tail`.

Co-authored-by: Oleksii Trekhleb <trehleb@gmail.com>