Do not look up path twice in dirImageSource.GetBlob

In principle this may return incorrect results if the underlying file is
replaced between the two calls, in practice computing and looking up the
path twice is just unnecessary overhead.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

directory/directory_src.go

@@ -38,7 +38,7 @@ func (s *dirImageSource) GetBlob(digest string) (io.ReadCloser, int64, error) {
 	if err != nil {
 		return nil, 0, nil
 	}
-	fi, err := os.Stat(s.ref.layerPath(digest))
+	fi, err := r.Stat()
 	if err != nil {
 		return nil, 0, nil
 	}