remove useless double lock (and fix possible race?)

cli/responseemitter.go

@@ -87,13 +87,13 @@ func (re *responseEmitter) isClosed() bool {
 }
 
 func (re *responseEmitter) Close() error {
-	if re.isClosed() {
-		return errors.New("closing closed responseemitter")
-	}
-
 	re.wLock.Lock()
 	defer re.wLock.Unlock()
 
+	if re.closed {
+		return errors.New("closing closed responseemitter")
+	}
+
 	re.ch <- re.exit
 	close(re.ch)