leds: trigger: ledtrig-backlight: Fix invalid memory access in fb_event notification callback

fb_notifier_callback is called on any event fired by
fb_notifier_call_chain. Events may, or may not contain some data
(fb_event.data). In case of FB_EVENT_BLANK fb_event.data contains a
pointer to an integer holdingthe blank state. The Problem is, that in
ledtrig-backlight.c - fb_notifier_callback the pointer to blank state
is dereferenced BEFORE the event-type is checked.

Obviously this leads to problems with other events than FB_EVENT_BLANK,
where fb_event.data is undefined or NULL. It seems, that this problem
existed ever since the driver was added.

Like in drivers/video/backlight/backlight.c line 43 I would suggest to
return immediately on events other than FB_EVENT_BLANK.

Signed-off-by: Manfred Schlaegl <manfred.schlaegl@gmx.at>
Signed-off-by: Bryan Wu <cooloney@gmail.com>

drivers/leds/trigger/ledtrig-backlight.c

@@ -36,26 +36,28 @@ static int fb_notifier_callback(struct notifier_block *p,
 					struct bl_trig_notifier, notifier);
 	struct led_classdev *led = n->led;
 	struct fb_event *fb_event = data;
-	int *blank = fb_event->data;
-	int new_status = *blank ? BLANK : UNBLANK;
+	int *blank;
+	int new_status;
 
-	switch (event) {
-	case FB_EVENT_BLANK:
-		if (new_status == n->old_status)
-			break;
+	/* If we aren't interested in this event, skip it immediately ... */
+	if (event != FB_EVENT_BLANK)
+		return 0;
 
-		if ((n->old_status == UNBLANK) ^ n->invert) {
-			n->brightness = led->brightness;
-			__led_set_brightness(led, LED_OFF);
-		} else {
-			__led_set_brightness(led, n->brightness);
-		}
+	blank = fb_event->data;
+	new_status = *blank ? BLANK : UNBLANK;
 
-		n->old_status = new_status;
+	if (new_status == n->old_status)
+		return 0;
 
-		break;
+	if ((n->old_status == UNBLANK) ^ n->invert) {
+		n->brightness = led->brightness;
+		__led_set_brightness(led, LED_OFF);
+	} else {
+		__led_set_brightness(led, n->brightness);
 	}
 
+	n->old_status = new_status;
+
 	return 0;
 }