Add NULL check
Add NULL check

vn_state could be not set if the VN notify happens prior to the VMI notify.

Change-Id: I87f6a90bf475fbe418945cd2f3465cdcd6fe06e8
closes-bug: #1764037

eonpatapon/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller