problem with uneven slides , when slideToScroll is set

The problem can be seem in the sandbox [HERE](https://codesandbox.io/s/react-slick-playground-ovwxk?file=/index.js)

Basically with the following settings:

```
const settings = {
slidesToShow: 1,
slidesToScroll: 3,
infinite: false,
};
// keep in mind slideCount is 14 in my example
```

The following occurs:
Click next 1st time :- moves to slide 4 - intended behaviour
Click next 2nd time :- moves to slide 7 - intended behaviour
Click next 3rd time :- moves to slide 10 - intended behaviour
Click next 4th time :- moves to slide 13 - intended behaviour
Click next 5th time :- moves to slide 16 (non existent slide) - non-intended behaviour

P.S. not sure why somebody would have these settings as its a bit odd , nevertheless the issue exists and would be great to have it fixed. I have a solution in mind and could raise a PR , let me know.