[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>