Close the listener if it has being initialized

shapeshifter.go

@@ -34,8 +34,11 @@ func (ss *ShapeShifter) Open() error {
 }
 
 func (ss *ShapeShifter) Close() error {
+	if ss.ln != nil {
 		return ss.ln.Close()
 	}
+	return nil
+}
 
 func (ss ShapeShifter) clientAcceptLoop() error {
 	for {