clocksource: Prevent NULL pointer dereference

Writing a zero length string to sys/.../current_clocksource will cause
a NULL pointer dereference if the clock events system is in one shot
(highres or nohz) mode.

Pointed-out-by: Dan Carpenter <[e-mail]>
LKML-Reference: <alpine.DEB.2.00.0907191545580.12306@bicker>
Signed-off-by: Thomas Gleixner <[e-mail]>

kernel/time/clocksource.c

···
 	 * Check to make sure we don't switch to a non-highres capable
 	 * clocksource if the tick code is in oneshot mode (highres or nohz)
 	 */
-	if (tick_oneshot_mode_active() &&
+	if (tick_oneshot_mode_active() && ovr &&
 	    !(ovr->flags & CLOCK_SOURCE_VALID_FOR_HRES)) {
 		printk(KERN_WARNING "%s clocksource is not HRT compatible. "
 			"Cannot switch while in HRT/NOHZ mode\n", ovr->name);