Stack Pop

As you should know by now, items go on and off the stack from the same end. (last in, first out).

Don't worry, the pop method is much simpler than the push method in our case, because we don't have to worry about resizing the stack.

Assignment

Complete the stack_pop function.

1. If the stack has no elements, return NULL.
2. Decrement the stack's count.
3. Return the top element of the stack (the count-th element in the array).