iface: use interface values only when valid
iface: use interface values only when valid

https://bugzilla.redhat.com/show_bug.cgi?id=690416

gloryleague/avahi,heftig/avahi-1,Kisensum/xmDNS-avahi,Distrotech/avahi,heftig/avahi,lathiat/avahi,gloryleague/avahi,Distrotech/avahi,lathiat/avahi,Kisensum/xmDNS-avahi,Kisensum/xmDNS-avahi,gloryleague/avahi,heftig/avahi-1,lathiat/avahi,catta-x/catta,heftig/avahi,catta-x/catta,gloryleague/avahi,Distrotech/avahi,everbase/catta,heftig/avahi,heftig/avahi,heftig/avahi-1,catta-x/catta,gloryleague/avahi,Distrotech/avahi,Kisensum/xmDNS-avahi,heftig/avahi-1,heftig/avahi-1,heftig/avahi,Distrotech/avahi,Distrotech/avahi,lathiat/avahi,Kisensum/xmDNS-avahi,everbase/catta,lathiat/avahi,gloryleague/avahi,Kisensum/xmDNS-avahi,heftig/avahi-1,lathiat/avahi,heftig/avahi,everbase/catta