close underlying writer firstly during async writer closing

Author: phuslu

async.go

@@ -30,13 +30,13 @@ type AsyncWriter struct {
 
 // Close implements io.Closer, and closes the underlying Writer.
 func (w *AsyncWriter) Close() (err error) {
-	w.ch <- nil
-	err = <-w.chClose
 	if closer, ok := w.Writer.(io.Closer); ok {
 		if err1 := closer.Close(); err1 != nil {
 			err = err1
 		}
 	}
+	w.ch <- nil
+	err = <-w.chClose
 	return
 }