selinux: fix potential memleak in selinux_add_opt()

This patch try to fix potential memleak in error branch.

Fixes: ba641862 ("selinux: new helper - selinux_add_opt()")
Signed-off-by: Bernard Zhao <bernard@vivo.com>
[PM: tweak the subject line, add Fixes tag]
Signed-off-by: Paul Moore <paul@paul-moore.com>