fix Infinite recursion inner 2005 (#3498)

src/main/java/com/actiontech/dble/backend/mysql/nio/handler/KillConnectionHandler.java

@@ -100,6 +100,7 @@ public class KillConnectionHandler implements ResponseHandler {
     @Override
     public void connectionClose(AbstractService service, String reason) {
         AlertUtil.alertSelf(AlarmCode.KILL_BACKEND_CONN_FAIL, Alert.AlertLevel.NOTICE, "get killer connection " + service.toString() + " failed: connectionClosed", null);
+        toKilled.getBackendService().setResponseHandler(null);
         toKilled.close("exception:" + reason);
     }
 }