Fix panic on promote after demote
Fix panic on promote after demote

Signed-off-by: Tonis Tiigi <c2470c48b2d3312d61f94f18d3a1cd113d1915ad@gmail.com>
AliyunContainerService/swarmkit,docker/swarmkit,cezarsa/swarmkit,yp-engineering/swarmkit,elifa/swarmkit,allencloud/swarmkit,runshenzhu/swarmkit,cheyang/swarmkit,diogomonica/swarmkit,AliyunContainerService/swarmkit,jlhawn/swarmkit,diogomonica/swarmkit,docker/swarmkit,vasil-yordanov/swarmkit,sungwonh/swarmkit,aaronlehmann/swarmkit,alyyousuf7/swarmkit,Noyan759/swarmkit,RenaudWasTaken/swarmkit,dperny/swarmkit-1,vasil-yordanov/swarmkit,sungwonh/swarmkit,deljuven/swarmkit,Noyan759/swarmkit,yongtang/swarmkit,dperny/swarmkit-1,yongtang/swarmkit,cheyang/swarmkit,runshenzhu/swarmkit,alyyousuf7/swarmkit,aaronlehmann/swarmkit,allencloud/swarmkit,deljuven/swarmkit,jlhawn/swarmkit,elifa/swarmkit,cezarsa/swarmkit,RenaudWasTaken/swarmkit,fate-grand-order/swarmkit,fate-grand-order/swarmkit,yp-engineering/swarmkit