[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 #2 from KL <deco33000 at yandex dot com> ---
During the parsing stage, the compiler has this information.

It knows that we want the end of v.
So it can inject a comparison into the resulting code.

A segfault here is not acceptable for a language like C++. There is no trap for
the compiler here.

That comparison is cheap.