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/ref.c b/net/tipc/ref.c
index 6704a58..c38744c 100644
--- a/net/tipc/ref.c
+++ b/net/tipc/ref.c
@@ -148,7 +148,7 @@
 		reference = (next_plus_upper & ~index_mask) + index;
 		entry->data.reference = reference;
 		entry->object = object;
-		if (lock != 0)
+		if (lock != NULL)
 			*lock = &entry->lock;
 		spin_unlock_bh(&entry->lock);
 	}