[boost] mapped_file_source::is_open() is broken under windows Ticket #1232

From: http://svn.boost.org:80/trac/boost/ticket/1232 Under windows, mapped_file_impl::handle_ gets set to INVALID_HANDLE_VALUE when the file gets closed or on error. But mapped_file_source::is_open() incorrectly assumes that if the handle is invalid it will have a value of zero and it doesn't consider INVALID_HANDLE_VALUE a possibility. Obviously, adding INVALID_HANDLE_VALUE to mapped_file_source::is_open() would be poor design. A better way would be to explose a mapped_file_impl::is_valid() method that works as it should depending on the platform and have mapped_file_source::is_open() call it. Regards, Steven Burns