Assign ready channel to nil only if it is not nil
Assign ready channel to nil only if it is not nil

If the ready channel is already nil, it's not necessary to assign
it to nil.

Signed-off-by: Jin Xu <07997fd85dbc5b62e4dc8b2519cd4a51995c53ac@hotmail.com>

AliyunContainerService/swarmkit,cezarsa/swarmkit,cezarsa/swarmkit,fate-grand-order/swarmkit,elifa/swarmkit,docker/swarmkit,RenaudWasTaken/swarmkit,fate-grand-order/swarmkit,alyyousuf7/swarmkit,yongtang/swarmkit,yongtang/swarmkit,allencloud/swarmkit,cheyang/swarmkit,allencloud/swarmkit,vasil-yordanov/swarmkit,deljuven/swarmkit,yp-engineering/swarmkit,docker/swarmkit,jlhawn/swarmkit,RenaudWasTaken/swarmkit,sungwonh/swarmkit,jlhawn/swarmkit,elifa/swarmkit,dperny/swarmkit-1,aaronlehmann/swarmkit,diogomonica/swarmkit,diogomonica/swarmkit,alyyousuf7/swarmkit,AliyunContainerService/swarmkit,dperny/swarmkit-1,Noyan759/swarmkit,cheyang/swarmkit,sungwonh/swarmkit,yp-engineering/swarmkit,deljuven/swarmkit,vasil-yordanov/swarmkit,Noyan759/swarmkit,aaronlehmann/swarmkit