fix: assemble for external metadata generates segfault if invalid device found

fix: assemble for external metadata generates segfault if invalid device found

An attempt to invoke super_by_fd() on device that has
metadata_version="none" always matches super0 (as test_version is "").
In Assemble() it results in segfault when load_container is invoked
(=null for super0).
As of now load_container is only started if it points to valid pointer.

Signed-off-by: Przemyslaw Czarnowski <przemyslaw.hawrylewicz.czarnowski@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>