Use nullptr

[matthew-reynolds]

Part of ros-controls/ros_control#403. Corresponds to ros-controls/ros_control#405.

Use nullptr instead of 0 or NULL for null pointers.

Note: ackermann_steering_controller.cpp and diff_drive_controller.cpp contain the following snippet:

if (!link)
{
 ROS_ERROR("Link == NULL.");
  return false;
}

Here, I did not change the NULL to nullptr since it's in a string. Would you like this updated as well? "Link == nullptr" or perhaps "Link pointer is null"?

[bmagyar]

Let's make the above snippet print a more informative message. The rest of the PR looks good to me!

[matthew-reynolds]

Updated to "Link pointer is null." This isn't that much more informative, but additional information is given the surrounding context, and this is at least more human-friendly to read.

[bmagyar]

Thank you!