Add nil-receiver awareness to all Reader/Writer

backend/generic.go

@@ -115,6 +115,10 @@ func (l *blobReader) Read(p []byte) (int, error) {
 }
 
 func (l *blobReader) Close() error {
+	if l == nil {
+		return nil
+	}
+
 	if !l.closed {
 		err := l.f.Close()
 		l.closed = true

backend/sftp/sftp.go

@@ -554,6 +554,10 @@ func (r *SFTP) ID() string {
 
 // Close closes the sftp connection and terminates the underlying command.
 func (s *SFTP) Close() error {
+	if s == nil {
+		return nil
+	}
+
 	s.c.Close()
 	// TODO: add timeout after which the process is killed
 	return s.cmd.Wait()

backend/writer.go

@@ -24,6 +24,10 @@ func NewHashAppendWriter(w io.Writer, h hash.Hash) *HashAppendWriter {
 }
 
 func (h *HashAppendWriter) Close() error {
+	if h == nil {
+		return nil
+	}
+
 	if !h.closed {
 		h.closed = true