libstdc++/10097: filebuf::underflow drops characters.

[Pétur Runólfsson]

>     Confirmed. Pétur, looks like this makes for a good testcase
>     for your suggestion to add:
>     
>     if (gptr() < egptr())
>       return *gptr();
>     
>     on top of underflow, right?

Yes.

Petur