utimens (fdutimens): ignore a negative FD, per contract

utimens (fdutimens): ignore a negative FD, per contract

* lib/utimens.c (fdutimens) [HAVE_FUTIMENS]: Call futimens only
when we have a valid file descriptor.  Otherwise, using a brand
new glibc (with just-patched futimens that now fails with EBADF)
would cause this function to fail with ENOSYS.
Reported by Guillaume Ayoub in http://bugs.debian.org/563726.
See also http://bugzilla.redhat.com/552320.

Signed-off-by: Eric Blake <ebb9@byu.net>
(cherry picked from commit 41c44a66101c6c14b4e7f151247d517579760ba9)