Detect if include/cctz/time_zone.h:join_seconds() would overflow the output time_point

[devbww]

template <typename Rep, std::intmax_t Denom>
bool join_seconds(
    const time_point<seconds>& sec, const femtoseconds& fs,
    time_point<std::chrono::duration<Rep, std::ratio<1, Denom>>>* tpp)

needs to return false when its result is unrepresentable, and without invoking UB.

[devjgm]

I don't see this function in the code. Link?

[devbww]

Yeah ... I have to create the issue before merging the code so that I can refer to the issue from the code. So, soon.