appletalk: Fix potential NULL pointer dereference in unregister_snap_client

register_snap_client may return NULL, all the callers
check it, but only print a warning. This will result in
NULL pointer dereference in unregister_snap_client and other
places.

It has always been used like this since v2.6

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
