[Bug libstdc++/106175] std::prev and next should check for equality with std::begin

[deco33000 at yandex dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106175

--- Comment #3 from KL <deco33000 at yandex dot com> ---
Please note, that comparison should only occur when you use next, prev or
advance.
It is useless for normal operations using end() directly. So no cost here.