[ACPI] Fix Null pointer deref in video/lcd/brightness

http://bugzilla.kernel.org/show_bug.cgi?id=5571

Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Yu Luming <luming.yu@gmail.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/video.c

@@ -812,7 +812,7 @@ acpi_video_device_write_brightness(struct file *file,
 
 	ACPI_FUNCTION_TRACE("acpi_video_device_write_brightness");
 
-	if (!dev || count + 1 > sizeof str)
+	if (!dev || !dev->brightness || count + 1 > sizeof str)
 		return_VALUE(-EINVAL);
 
 	if (copy_from_user(str, buffer, count))