Injecting route with missing next-hop value causes exception in reachability topology builder

Next-hop is a well-known mandatory attribute and should be present when injecting IPv4/v6 unicast routes. However bad input where the value is missing causes exception to be thrown while updating IPv4/v6 reachability topology

NPE is thrown in else block of AbstractReachabilityTopologyBuilder#advertizingNode when nh is null

private NodeId advertizingNode(final Attributes attrs) {
final CNextHop nh = attrs.getCNextHop();
if (nh instanceof Ipv4NextHopCase)

{ final Ipv4NextHop ipv4 = ((Ipv4NextHopCase) nh).getIpv4NextHop(); return new NodeId(ipv4.getGlobal().getValue()); }

else if (nh instanceof Ipv6NextHopCase)

{ final Ipv6NextHop ipv6 = ((Ipv6NextHopCase) nh).getIpv6NextHop(); return new NodeId(ipv6.getGlobal().getValue()); }

else {
LOG.warn("Unhandled next hop class {}", nh.getImplementedInterface());
return null;
}
}