Fixed node(atIndex:) method to be the same in LinkedList.swift file.

Author: yossan

Linked List/LinkedList.playground/Contents.swift

@@ -72,6 +72,8 @@ public final class LinkedList<T> {
   /// - Returns: Optional LinkedListNode
   public func node(atIndex index: Int) -> Node {
     assert(head != nil, "List is empty")
+    assert(index >= 0, "index must be greater than 0")
+    
     if index == 0 {
         return head!
     } else {