Unblock netvirt CSIT issues

Unblock netvirt CSIT issues

Issue:
======
New exceptions found: 3
[{'warnerr_list': [6, 38], 'lines': ['2020-07-17T05:22:39,604 | ERROR |
NeutronFloatingToFixedIpMappingChangeListener-0 |
NeutronFloatingToFixedIpMappingChangeListener | 359 -
org.opendaylight.netvirt.neutronvpn-impl - 0.11.0.SNAPSHOT |
addToFloatingIpInfo failed for floating IP: 10.10.10.187 ',
'java.lang.NullPointerException: null', '\tat
org.opendaylight.netvirt.neutronvpn.NeutronFloatingToFixedIpMappingChangeListener
.addToFloatingIpInfo(NeutronFloatingToFixedIpMappingChangeListener.java:167)
[bundleFile:?]', '\tat
org.opendaylight.netvirt.neutronvpn.NeutronFloatingToFixedIpMappingChangeListener
.update(NeutronFloatingToFixedIpMappingChangeListener.java:121)
[bundleFile:?]', '\tat

https://logs.opendaylight.org/sandbox/vex-yul-odl-jenkins-2/srini-netvirt-csit-1node-0cmb-1ctl-2cmp-openstack-rocky-gate-stateful-aluminium/16/robot-plugin/log_full.html.gz

Solution:
=========
Provided null check in appropriate place to avoid NPE

Signed-off-by: Karthikeyan Krishnan <karthikeyangceb007@gmail.com>
Change-Id: I812463516f7fe773e47fcd80d169880e5519a1fa