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/zone.c b/net/tipc/zone.c
index 114e173..3506f85 100644
--- a/net/tipc/zone.c
+++ b/net/tipc/zone.c
@@ -82,7 +82,7 @@
 
 	assert(c_ptr->addr);
 	assert(c_num <= tipc_max_clusters);
-	assert(z_ptr->clusters[c_num] == 0);
+	assert(z_ptr->clusters[c_num] == NULL);
 	z_ptr->clusters[c_num] = c_ptr;
 }