tipc: replace 0 by NULL for pointers

Fix sparse warning:
net/tipc/link.c:1924:40: warning: Using plain integer as NULL pointer

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>