agent: don't handle registered on error
agent: don't handle registered on error

There is non-zero chance that select will choose session.errs before
registered if they occured at the same time. So it's possible to get
sequence session.errs -> registered -> session.closed. In that case
backoff will be zero and rand.Int63n will panic.

Fix #1131

Signed-off-by: Alexander Morozov <2043cda4df124b840b98ee1c4307c199042efdf7@gmail.com>

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