cpuidle: Prevent null pointer dereference in cpuidle_coupled_cpu_notify

When a kernel is built to support multiple hardware types it's possible
that CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED is set but the hardware the
kernel is run on doesn't support cpuidle and therefore doesn't load a
driver for it. In this case, when the system is shut down,
cpuidle_coupled_cpu_notify() gets called with cpuidle_devices set to
NULL. There are quite possibly other circumstances where this
situation can also occur and we should check for it.
Signed-off-by: Jon Medhurst <tixy@linaro.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

drivers/cpuidle/coupled.c

@@ -693,7 +693,7 @@ static int cpuidle_coupled_cpu_notify(struct notifier_block *nb,
 	mutex_lock(&cpuidle_lock);
 
 	dev = per_cpu(cpuidle_devices, cpu);
-	if (!dev->coupled)
+	if (!dev || !dev->coupled)
 		goto out;
 
 	switch (action & ~CPU_TASKS_FROZEN) {