Fix bug "...has already joined the endpoint"
Fix bug "...has already joined the endpoint"

- In case of sandboxAdd() failure, drive.Leave() call
  in first executed defer reset err to nil. Secondly
  executed defer in charge of resetting ep.container to nil
  will not get executed.

Signed-off-by: Alessandro Boch <5bd2eb5b6f60921d9a2b9cb6a0a86bb947f3d361@docker.com>

thaJeztah/docker,jthelin/docker,docker/docker,djs55/docker,thaJeztah/docker,bearstech/moby,konstruktoid/docker-upstream,jameszhan/docker,cpuguy83/docker,nalind/docker,nawawi/docker,mahak/docker,AkihiroSuda/docker,mahak/docker,mahak/docker,djs55/docker,konstruktoid/docker-upstream,bearstech/moby,bearstech/moby,nawawi/docker,moul/docker,cpuguy83/docker,Mokolea/docker,Ramzec/docker,Ramzec/docker,moby/moby,moul/docker,docker/docker,yosifkit/docker,nalind/docker,paralin/docker,nawawi/docker,paralin/docker,jameszhan/docker,paralin/docker,Mokolea/docker,nalind/docker,djs55/docker,jimjag/docker,AkihiroSuda/docker,mahak/docker,yosifkit/docker,jameszhan/docker,cpuguy83/docker,thaJeztah/docker,AkihiroSuda/docker,gnailuy/docker,trebonian/docker,paralin/docker,flavio/docker,thaJeztah/docker,jimjag/docker,flavio/docker,Ramzec/docker,flavio/docker,yosifkit/docker,trebonian/docker,trebonian/docker,jimjag/docker,flavio/docker,moby/moby,trebonian/docker,konstruktoid/docker-upstream,vdemeester/docker,Mokolea/docker,vdemeester/docker,AkihiroSuda/docker,nawawi/docker,vdemeester/docker,docker/docker,moul/docker,gnailuy/docker,moby/moby,moul/docker,gnailuy/docker,cpuguy83/docker,Ramzec/docker,konstruktoid/docker-upstream,yosifkit/docker,Mokolea/docker,jthelin/docker,moby/moby,nalind/docker,jimjag/docker,gnailuy/docker,bearstech/moby