[Patch] Minor clean up of basic_stringbuf::pbackfail

[Paolo Carlini]

Benjamin Kosnik wrote:

>Seems fine. Did you/are you going to do consistency checks for
>stringbuf/filebuf for pbackfail? Or is this already done in the existing
>testsuite? (Ie make sure that filebuf/stringbuf have the same functional
>tests, and if their behavior diverges, make sure that it's tested so we
>can track expected behavior.)
>
I'm not sure is already done. I suspect we don't have a sufficient 
number of testcases in this area.

Will take care of that.

Thanks for the heads up!
Paolo.