Fix the interface_is_vlan() function.
Fix the interface_is_vlan() function.

It now works when only QinQ VLANs exist in the system.

ptorsten/pfsense,NewEraCracker/pfsense,phil-davis/pfsense,pfsense/pfsense,NewEraCracker/pfsense,dennypage/pfsense,pfsense/pfsense,dennypage/pfsense,phil-davis/pfsense,ptorsten/pfsense,phil-davis/pfsense,dennypage/pfsense,phil-davis/pfsense,pfsense/pfsense,dennypage/pfsense,ptorsten/pfsense,NewEraCracker/pfsense,NewEraCracker/pfsense,pfsense/pfsense