While implementing ifstream::setbuf() it turned out that filebuf::setbuf(),
behaves a bit different on Windows than our implementation.

It seems to reimplement streambuf::setbuf() with a very minor difference - it
doesn't ever set unbuffered to 0 and allows to change the underlying buffer as
long as the file is not opened.
Signed-off-by: Arkadiusz Hiler <ahiler@codeweavers.com>
Signed-off-by: Piotr Caban <piotr@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>