rtc: verify a critical argument to rtc_update_irq() before using it

This small addition to the core simplifies code in the drivers and makes
them more robust when handling shared IRQs.

Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/rtc/interface.c

@@ -584,6 +584,9 @@ enum hrtimer_restart rtc_pie_update_irq(struct hrtimer *timer)
 void rtc_update_irq(struct rtc_device *rtc,
 		unsigned long num, unsigned long events)
 {
+	if (unlikely(IS_ERR_OR_NULL(rtc)))
+		return;
+
 	pm_stay_awake(rtc->dev.parent);
 	schedule_work(&rtc->irqwork);
 }