Robot overturn when I controlled with the keyboard

edit

I've just started with ROS and Gazebo and I'm learning how and where to implement my code to control a robot in Gazebo.

I'm trying to control robot Pioneer 2DX robot in Gazebo using ROS.

To do it, I have followed the tutorial: Controlling a differential drive robot in Gazebo from ROS. And to control it with the keyboard, I run this:

rosrun turtlesim turtle_teleop_key /turtle1/cmd_vel:=/pioneer2dxCamera/cmd_vel

But, there is a problem, because if I press up key, and then down key the robot overturn:

I think, the problem is because the linear velocity changes from 2 to -2. Look:

linear: 
  x: 2.0
  y: 0.0
  z: 0.0
angular: 
  x: 0.0
  y: 0.0
  z: 0.0
---
linear: 
  x: -2.0
  y: 0.0
  z: 0.0
angular: 
  x: 0.0
  y: 0.0
  z: 0.0
---

I have decided to modify program turtle_teleop_key to store current linear and angular velocity, and, if linear velocity is 2 and I press down key, linear velocity will 0 instead of -2.

My question is:

Is this the correct place to do it or do I have to do it in the diff drive plugin?

By the way, I'm using this plugin, libgazebo_ros_diff_drive.so, to control the robot.

Yes, I know that change linear velocity to 2 to -2 will flip a real robot too. I'm not asking that.