Cygwin: fstat_helper: always use handle in call to get_file_attribute

Cygwin: fstat_helper: always use handle in call to get_file_attribute

Previously, the call to get_file_attribute for FIFOs set the first
argument to NULL instead of the handle h returned by get_stat_handle,
thereby forcing the file to be opened for fetching the security
descriptor in get_file_sd().  This was done because h might have been
a pipe handle rather than a file handle, and its permissions would not
necessarily reflect those of the file.

That situation can no longer occur with the new fhandler_fifo::fstat
introduced in the previous commit.