fix: nil pointer dereference when using "docker volume ls"
fix: nil pointer dereference when using "docker volume ls"

Signed-off-by: Terry Chen <579926d59a9c3e22ce3647a4283e0e0b5f767daf@gmail.com>

libopenstorage/openstorage,pault84/openstorage,pault84/openstorage,libopenstorage/openstorage,pault84/openstorage,libopenstorage/openstorage