[TIPC]: Can now list multicast link on an isolated network node

This patch fixes a minor bug that prevents "tipc-config -l" from
displaying the multicast link if a TIPC node has never successfully
established at least one unicast link.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Per Liden <per.liden@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>