[Bug libstdc++/64814] std::copy_n advances InputIterator one *less* time than necessary.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64814

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. libc++ changed its copy_n with
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20110221/039404.html
and then libstdc++ did  the same in PR 50119