lldp: let lldp_neighbor_new() also create invalid objects

lldp: let lldp_neighbor_new() also create invalid objects

When we receive an update for a certain neighbor, the update
might be invalid and we want to reject it. However, we still
must create an invalid object to compare whether the update
causes a remove of a previously valid neighbor.

Let lldp_neighbor_new() create an instance as long as the
id fields are present.