robot: check for root_link_ before use (#1041)
robot: check for root_link_ before use (#1041)

A couple of lines below the code accesses link->getChildJointNames()
without testing "link". The additional test guarantees the link is set
before working with it.

I noticed that when adding support to MoveIt to correctly initialize
an empty robot model after printing an error.
ros-visualization/rviz,ros-visualization/rviz,ros-visualization/rviz,ros-visualization/rviz,ros-visualization/rviz