tipc: fix integer as NULL pointer sparse warnings in tipc

net/tipc/cluster.c:145:2: warning: Using plain integer as NULL pointer
net/tipc/link.c:3254:36: warning: Using plain integer as NULL pointer
net/tipc/ref.c:151:15: warning: Using plain integer as NULL pointer
net/tipc/zone.c:85:2: warning: Using plain integer as NULL pointer

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/tipc/cluster.c b/net/tipc/cluster.c
index 95b3739..4bb3404 100644
--- a/net/tipc/cluster.c
+++ b/net/tipc/cluster.c
@@ -142,7 +142,7 @@
 		max_n_num = tipc_highest_allowed_slave;
 	assert(n_num > 0);
 	assert(n_num <= max_n_num);
-	assert(c_ptr->nodes[n_num] == 0);
+	assert(c_ptr->nodes[n_num] == NULL);
 	c_ptr->nodes[n_num] = n_ptr;
 	if (n_num > c_ptr->highest_node)
 		c_ptr->highest_node = n_num;