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

[kristian.spangsege at gmail dot com via Gcc-bugs]

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

--- Comment #14 from Kristian Spangsege <kristian.spangsege at gmail dot com> 
---
Is the underscore intended before `Guard` in `explicit _Guard(__c_locale) { }`
for the GLIBC <= 2.2 case?

Besides that, the full patch looks correct to me.