tar: populate Entry tree under a common root

Resolves #56. Now, the Entry tree will be populated
under a common root (if necessary), so that a manifest can
be accurately generate from a tar file that has been
created using multiple directories.

Signed-off-by: Stephen Chung <schung@redhat.com>

check.go

@@ -42,7 +42,6 @@ func Check(root string, dh *DirectoryHierarchy, keywords []string) (*Result, err
 		return nil, err
 	}
 	sort.Sort(byPos(creator.DH.Entries))
-
 	var result Result
 	for i, e := range creator.DH.Entries {
 		switch e.Type {
@@ -118,6 +117,9 @@ func TarCheck(tarDH, dh *DirectoryHierarchy, keywords []string) (*Result, error)
 			break
 		}
 	}
+	if tarRoot == nil {
+		return nil, fmt.Errorf("root of archive could not be found")
+	}
 	tarRoot.Next = &Entry{
 		Name: "seen",
 		Type: CommentType,