tipc: eliminate KASAN warning

The following warning was reported by syzbot on Oct 24. 2017:
KASAN: slab-out-of-bounds Read in tipc_nametbl_lookup_dst_nodes

This is a harmless bug, but we still want to get rid of the warning,
so we swap the two conditions in question.

Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>