Fix `String.slice error`

When you use 0 as the first range number, negative as the second number in `String.slice/2` , it returns `nil`,
but I think it's wrong.

    iex(76)> String.slice("string", 0..-2)
    nil # you see here is the issue, it shouldn't return `nil`
    iex(77)> String.slice("string", 1..-2)
    "trin"
    iex(78)> String.slice("string", -5..-2)
    "trin"