cpuidle: Check the result of cpuidle_get_driver() against NULL

commit 3b9c10e98021e1f92e6f8c7ce1778b86ba68db10 upstream.

If the current CPU has no cpuidle driver, drv will be NULL in
cpuidle_driver_ref().  Check if that is the case before trying
to bump up the driver's refcount to prevent the kernel from
crashing.

[rjw: Subject and changelog]
Signed-off-by: Daniel Fu <danifu@nvidia.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/cpuidle/driver.c

@@ -251,7 +251,8 @@ struct cpuidle_driver *cpuidle_driver_ref(void)
 	spin_lock(&cpuidle_driver_lock);
 
 	drv = cpuidle_get_driver();
-	drv->refcnt++;
+	if (drv)
+		drv->refcnt++;
 
 	spin_unlock(&cpuidle_driver_lock);
 	return drv;