Fix a bug which caused creation of empty images (and volumes) to crash. FIxes #995.

archive.go

@@ -108,7 +108,9 @@ func TarFilter(path string, compression Compression, filter []string) (io.Reader
 //  identity (uncompressed), gzip, bzip2, xz.
 // FIXME: specify behavior when target path exists vs. doesn't exist.
 func Untar(archive io.Reader, path string) error {
-
+	if archive == nil {
+		return fmt.Errorf("Empty archive")
+	}
 	bufferedArchive := bufio.NewReaderSize(archive, 10)
 	buf, err := bufferedArchive.Peek(10)
 	if err != nil {

image.go

@@ -92,10 +92,12 @@ func StoreImage(img *Image, layerData Archive, root string, store bool) error {
 		defer file.Close()
 		layerData = file
 	}
-
+	// If layerData is not nil, unpack it into the new layer
+	if layerData != nil {
 		if err := Untar(layerData, layer); err != nil {
 			return err
 		}
+	}
 
 	return StoreSize(img, root)
 }