opendir-safer.c: don't clobber errno; don't close negative FD

opendir-safer.c: don't clobber errno; don't close negative FD

* lib/opendir-safer.c (opendir_safer):
[HAVE_FDOPENDIR || GNULIB_FDOPENDIR]: Don't close a negative
file descriptor, and more importantly, don't clobber the
offending errno value with EINVAL.  Before, upon failure
of dup_safer, we would pass the negative file descriptor to
fdopendir, which would clobber errno.