Fix for PR 6745, 8071, 8127: __copy_streambufs in odd cases

[Benjamin Kosnik]

Nice.

> I don't have a GCC copyright assignment filed.  Is this patch substantial
> enough to require one?  (I don't think the test case requires a copyright
> assignment, yes?)

You're fine. Of course, you are encouraged to continue contributing. If
this has any appeal, I suggest filing a copyright assignment. The
details are on the libstdc++ docs page, at the bottom.

> 2002-10-07  Jonathan Lennox  <lennox@cs.columbia.edu>
> 
> 	PR libstdc++/8071, libstdc++/8127, c++/6745
> 	* streambuf.tcc (__copy_streambufs): Handle cases where
> 	__sbin->in_avail() returns 0, or where __sbin doesn't set gptr().