Cygwin: fstat_helper: always use handle in call to get_file_attribute

Cygwin: fstat_helper: always use handle in call to get_file_attribute

When fhandler_base::fstat_helper is called, the handle h returned by
get_stat_handle() should be pc.handle() and should be safe to use for
getting the file information.  Previously, the call to
get_file_attribute() for FIFOs set the first argument to NULL instead
of h, thereby forcing the file to be opened for fetching the security
descriptor in get_file_sd().