Unchecked lower bound warning on `regex_traits_defaults.hpp::get_default_error_string()`

[ealva]

Flagged by MSVC as C33010 (there might be others in the same file).

[oldnewthing]

If error_type is negative, then the code indexes beyond the beginning of the array. The range check needs to check for "less than error_ok" or cast to uint before checking the upper bound.