NETVIRT-1252: NPE while creating internet VPN interface

NETVIRT-1252: NPE while creating internet VPN interface

Due to an NPE introduced as part of
https://git.opendaylight.org/gerrit/#/c/57411/, VPN interfaces belonging to
internet based VPNs are not getting created. This is part of a DJC job which
creates IETF interface for the neutron port as well. Due to this failure,
table 0 entry is ultimately not getting realized.

This commit fixes the same.

Change-Id: I8cdcb273c47c2627583cd367fa4dbfba446298a2
Signed-off-by: Abhinav Gupta <abhinav.gupta@ericsson.com>