Don't try to "validate" a non-existing timeval.

settime() with a NULL timeval is silly but legal.

Noticed by Dave Jones <davej@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
[chrisw: seclvl only]
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>