selinux: fix error return code in cond_read_list()

Fix to return negative error code -ENOMEM from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: 60abd318 ("selinux: convert cond_list to array")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>