Possible integral overflow in sum_string_vector

[trokhymchuk]

I believe there is a possible UB in the sum_string_vector function (CLI11/include/CLI/TypeTools.hpp:1630), in the 1652-1658 lines:

        if(val <= static_cast<double>((std::numeric_limits<std::int64_t>::min)()) ||
           val >= static_cast<double>((std::numeric_limits<std::int64_t>::max)()) ||
           std::ceil(val) == std::floor(val)) {
            output = detail::value_string(static_cast<int64_t>(val));
        } else {
            output = detail::value_string(val);
        }

the val gets casted to the std::int64_t if it is less than min or greater than max value.