Handles the case where a wire in the wires table has an 'Unused' role
(which is how logical wires begin life -- then once their underlying
physical links are built into vlans, snmpit changes their wires.role
changes to 'Node').  This change ensures undef ports (which is what is
returned for wires entries with 'Unused' roles) don't make it into the
getVlanPorts list, where they cause snmpit to die, of course.