Since rc_unregister_device() frees its argument, the subsequently
call to rc_free_device() on the same variable will cause a double
free bug. Fix by set argument to NULL, thus when fall through to
rc_free_device(), nothing will be done there.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>