core: IndexOutOfBounds exception during RestoreAllSnapshotsCommand

When running the command for diskless snapshots an IndexOutOfBounds
exception  might occur as there is an attempt to access the first
snapshot disk while there are no disks for the snapshot.

Change-Id: I1e56e6f266c22e59f33dde4ee4f4921e6e807e43
Bug-Url:https://bugzilla.redhat.com/show_bug.cgi?id=892724
Signed-off-by: Liron Aravot <laravot@redhat.com>