set shutdownWG to nil after stopBGP finished
set shutdownWG to nil after stopBGP finished

otherwise gobgp will panic in handlefsmMsg when handle error condition

Signed-off-by: Duan Jiong <b1d557e52dc56c138eb855d08eb69a4e274ec7bc@yunify.com>

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