Fix a bug in interface_is_vlan(), only check QinQ interfaces when they exist.
Fix a bug in interface_is_vlan(), only check QinQ interfaces when they exist.

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