bugfix: fix a panic when delete neighborset (all) by name directly
bugfix: fix a panic when delete neighborset (all) by name directly

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