Fixing edge cases in Enum.slide/3

[sabiwara]

The first bug can happen on empty lists:

iex> Enum.slide([], 1, 2)
** (FunctionClauseError) no function clause matching in Enum.slide_list_start/4

    The following arguments were given to Enum.slide_list_start/4:

        # 1
        []

        # 2
        1

        # 3
        2

        # 4
        2

    Attempted function clauses (showing 2 out of 2):

        defp slide_list_start([h | t], start, middle, last) when start > 0 and start <= middle and middle <= last
        defp slide_list_start(list, 0, middle, last)

    (elixir 1.13.4) lib/enum.ex:2665: Enum.slide_list_start/4

The second bug is on a weird edge case because an assumed invariant (start <= stop) isn't actually checked (but only happens for some precise values of the insertion index):

iex> Enum.slide([1, 2, 3, 4], 2..1//1, -3)
** (FunctionClauseError) no function clause matching in Enum.slide_list_start/4

    The following arguments were given to Enum.slide_list_start/4:

        # 1
        [1, 2, 3, 4]

        # 2
        1

        # 3
        2

        # 4
        1

    Attempted function clauses (showing 2 out of 2):

        defp slide_list_start([h | t], start, middle, last) when start > 0 and start <= middle and middle <= last
        defp slide_list_start(list, 0, middle, last)

    (elixir 1.13.4) lib/enum.ex:2665: Enum.slide_list_start/4

[whatyouhide]

Good catches @sabiwara 💟

[josevalim]

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