[Bug libstdc++/37475] codecvt::do_in/do_out functions return "ok" when the output sequence has zero length

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37475

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Whereas the GNU locale has that check inside a loop, which is never entered
when the destination buffer is zero sized, i.e. __to == __to_end

        if (__from_next < __from_end && __ret == ok)
          {
            if (__to_next < __to_end)
              {
                // XXX Probably wrong for stateful encodings
                __tmp_state = __state;
                ++__from_next;
                *__to_next++ = L'\0';
              }
            else
              __ret = partial;
          }