appletalk: Set error code if register_snap_client failed

commit c93ad1337ad06a718890a89cdd85188ff9a5a5cc upstream.

If register_snap_client fails in atalk_init,
error code should be set, otherwise it will
triggers NULL pointer dereference while unloading
module.

Fixes: 9804501fa122 ("appletalk: Fix potential NULL pointer dereference in unregister_snap_client")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>