fix bug pointed out by issue #2089
fix bug pointed out by issue #2089

fujita/gobgp,osrg/gobgp,osrg/gobgp,j-kato/gobgp,a16/gobgp,a16/gobgp,a16/gobgp,a16/gobgp,j-kato/gobgp,osrg/gobgp,a16/gobgp,fujita/gobgp,a16/gobgp,j-kato/gobgp,fujita/gobgp,a16/gobgp,fujita/gobgp,osrg/gobgp