[v3] char_traits error checking

[Benjamin Kosnik]

> In the above test the beg =3D=3D _InIter(0) test is almost never true.  And
> even if it would be, the fast path is to not throw an exception.=20
> Therefore the if () expressions should be written like this:
> 
>   if (__builtin_expect (__beg =3D=3D _InIter(0), 0))

Yep. I'll do this, thanks for the suggestion.

At some point, the library code should be audited for this.

-benjamin