Problem in 27_io/istream_sentry.cc (patch included)

[Benjamin Kosnik]

> If the function test2() of 27_io/istream_sentry.cc is run on a system that
> exposes the behaviour described in libstdc++/944, the while loop never
> terminates if eof is not processed correctly. I believe it is the
> correct handling of eof that the function test2() is checking. Therefore an
> explicit exit of the loop for a misbehaving implementation should be
> provided. I apologise if this is a bogus report.

ok

> Thanks very much for fixing the problem described in libstdc++/944.

thank you. I'll check this in.