[bug report] appletalk: Fix use-after-free in atalk_proc_exit

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

Hello YueHaibing,

This is a semi-automatic email about new static checker warnings.

The patch 6377f787aeb9: "appletalk: Fix use-after-free in 
atalk_proc_exit" from Mar 1, 2019, leads to the following Smatch 
complaint:

    net/appletalk/ddp.c:1952 atalk_init()
    error: we previously assumed 'ddp_dl' could be null (see line 1924)

net/appletalk/ddp.c
  1923		ddp_dl = register_snap_client(ddp_snap_id, atalk_rcv);
  1924		if (!ddp_dl)
                    ^^^^^^^
Check

  1925			printk(atalk_err_snap);
  1926	
  1927		dev_add_pack(&ltalk_packet_type);
  1928		dev_add_pack(&ppptalk_packet_type);
  1929	
  1930		rc = register_netdevice_notifier(&ddp_notifier);
  1931		if (rc)
  1932			goto out_sock;
  1933	
  1934		aarp_proto_init();
  1935		rc = atalk_proc_init();
  1936		if (rc)
  1937			goto out_aarp;
  1938	
  1939		rc = atalk_register_sysctl();
  1940		if (rc)
  1941			goto out_proc;
  1942	out:
  1943		return rc;
  1944	out_proc:
  1945		atalk_proc_exit();
  1946	out_aarp:
  1947		aarp_cleanup_module();
  1948		unregister_netdevice_notifier(&ddp_notifier);
  1949	out_sock:
  1950		dev_remove_pack(&ppptalk_packet_type);
  1951		dev_remove_pack(&ltalk_packet_type);
  1952		unregister_snap_client(ddp_dl);
                                       ^^^^^^
Unchecked dereference.

  1953		sock_unregister(PF_APPLETALK);
  1954	out_proto:

regards,
dan carpenter