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>