tipc: checking for NULL instead of IS_ERR()

The tipc_alloc_conn() function never returns NULL, it returns error
pointers, so I have fixed the check.

Fixes: 14c04493 ("tipc: add ability to order and receive topology events in driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>