FlowTable: Don't send RouteMod with unknown gateway
FlowTable: Don't send RouteMod with unknown gateway

Signed-off-by: Joe Stringer <da02367898403910c66f5d62c3a247888772ce8a@gmail.com>

ralph-mikera/RouteFlow-1,c3m3gyanesh/RouteFlow-OpenConfig,ralph-mikera/RouteFlow-1,arazmj/RouteFlow,arazmj/RouteFlow,arazmj/RouteFlow,CPqD/RouteFlow,routeflow/RouteFlow,arazmj/RouteFlow,c3m3gyanesh/RouteFlow-OpenConfig,srijanmishra/RouteFlow,ralph-mikera/RouteFlow-1,raphaelvrosa/RouteFlow,CPqD/RouteFlow,routeflow/RouteFlow,arazmj/RouteFlow,srijanmishra/RouteFlow,rsanger/RouteFlow,ralph-mikera/RouteFlow-1,srijanmishra/RouteFlow,raphaelvrosa/RouteFlow,c3m3gyanesh/RouteFlow-OpenConfig,arazmj/RouteFlow,raphaelvrosa/RouteFlow,srijanmishra/RouteFlow,raphaelvrosa/RouteFlow,raphaelvrosa/RouteFlow,c3m3gyanesh/RouteFlow-OpenConfig,rsanger/RouteFlow,raphaelvrosa/RouteFlow,routeflow/RouteFlow,CPqD/RouteFlow,routeflow/RouteFlow,CPqD/RouteFlow,rsanger/RouteFlow,ralph-mikera/RouteFlow-1,rsanger/RouteFlow,rsanger/RouteFlow,rsanger/RouteFlow,srijanmishra/RouteFlow,c3m3gyanesh/RouteFlow-OpenConfig,c3m3gyanesh/RouteFlow-OpenConfig,ralph-mikera/RouteFlow-1,routeflow/RouteFlow,CPqD/RouteFlow