configverify: T3992: fix KeyError in verify_address()

Reproducible by:
set interfaces bridge br0 member interface eth1.10
set interfaces ethernet eth1 vif 10 address 100.64.0.1/24

File "/usr/lib/python3/dist-packages/vyos/configverify.py", line 314, in verify_vlan_config

verify_address(vlan)

File "/usr/lib/python3/dist-packages/vyos/configverify.py", line 206, in verify_address

interface = config['ifname']

KeyError: 'ifname'