Mutex protect buffer access in standard renderer

standard_renderer.go

@@ -88,6 +88,9 @@ func (r *standardRenderer) listen() {
 
 // flush renders the buffer.
 func (r *standardRenderer) flush() {
+	r.mtx.Lock()
+	defer r.mtx.Unlock()
+
 	if r.buf.Len() == 0 || r.buf.String() == r.lastRender {
 		// Nothing to do
 		return
@@ -95,9 +98,6 @@ func (r *standardRenderer) flush() {
 
 	out := new(bytes.Buffer)
 
-	r.mtx.Lock()
-	defer r.mtx.Unlock()
-
 	// Clear any lines we painted in the last render.
 	if r.linesRendered > 0 {
 		for i := r.linesRendered - 1; i > 0; i-- {