linux-3.8: backport a leds-gpio.c crash fix (fixes #13721)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@36968 3c298f89-4303-0410-b956-a3cf2f4a3e73