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>

carlye566/libnetwork_eric,carlye566/libnetwork_eric