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/link.c b/net/tipc/link.c
index 1b17fec..cefa998 100644
--- a/net/tipc/link.c
+++ b/net/tipc/link.c
@@ -3251,7 +3251,7 @@
 		if ((mod(msg_seqno(buf_msg(l_ptr->last_out)) -
 			 msg_seqno(buf_msg(l_ptr->first_out)))
 		     != (l_ptr->out_queue_size - 1))
-		    || (l_ptr->last_out->next != 0)) {
+		    || (l_ptr->last_out->next != NULL)) {
 			tipc_printf(buf, "\nSend queue inconsistency\n");
 			tipc_printf(buf, "first_out= %x ", l_ptr->first_out);
 			tipc_printf(buf, "next_out= %x ", l_ptr->next_out);