[Patch] Fix libstdc++/9507

[Pétur Runólfsson]

Hi

> among the ton of PR filed by Pétur ;), this one appears pretty
> easy to analyze and fix.

Thanks :-)

> P.S. The original testcase is really involved compared to the
> underlying issue and I would rather prefer not adding it.

It might be easier to test this by deriving a class from std::filebuf
and override seekoff to always return pos_type(off_type(-1)).

Petur