slickNext/slickPrev not working as expected

The `slickPrev()` and `slickNext()` functions do not behave as expected if `initialSlide` is set to something other than `0`, and `infinite` is `false`. This behavior is only seen the first time the 👈 or 👉 button is pressed (i.e. after a page refresh).

With an `initialSlide` of `2`, the buttons behave as follows:
- Previous - Goes to slide 0
- Next - Goes to slide 1

Note: `goTo()` still works as expected

[CodeSandbox example](https://codesandbox.io/s/react-slick-bug-31r8d2)