Fix Integer.undigits/2 to validate negative out-of-range digits

[pnezis]

The validation was asymmetric - it rejected positive digits >= base but silently accepted negative digits <= -base. This fix adds the missing check for negative out-of-range digits.

iex> Integer.undigits([100])
** (ArgumentError) invalid digit 100 in base 10
    (elixir 1.18.4) lib/integer.ex:229: Integer.undigits/3
    iex:9: (file)
iex(9)> Integer.undigits([-100])
-100

[josevalim]

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