[libstdc++ patch] Fix basic_string::_S_construct null pointer check

[Paolo Carlini]

Phil Edwards wrote:

>I have a feeling these will come in handy in other files, but I can't
>think of a better place to put them right now.  In the meantime:
>
>2)  Please name the function __is_null_pointer or __null_pointer_p
>    or something like that, to make it clear this is a predicate function.
>    Reading the code where it was used made me think at first that the
>    function creates and returns a null pointer.
>
>3)  This should go in __gnu_cxx, not std.  Actually, this should go in
>    our internal-details namespace, but we haven't invented that name yet.
>
... modulo Phil's comments of course!

Paolo.