tipc: checking for NULL instead of IS_ERR()

The tipc_alloc_conn() function never returns NULL, it returns error
pointers, so I have fixed the check.

Fixes: 14c04493cb77 ("tipc: add ability to order and receive topology events in driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/server.c

@@ -504,7 +504,7 @@ bool tipc_topsrv_kern_subscr(struct net *net, u32 port, u32 type,
 	*(u32 *)&sub.usr_handle = port;
 
 	con = tipc_alloc_conn(tipc_topsrv(net));
-	if (!con)
+	if (IS_ERR(con))
 		return false;
 
 	*conid = con->conid;