EffortDisplay: Added a check to avoid segfaults when receiving a joint s...

...tate without efforts

We were using robot_state_publisher to test a custom GUI with an effortDisplay and a robotModelDisplay.
The effortDisplay was causing crashes because robot_state_publisher does not publish effort information. 
We tried in the normal Rviz and the behavior is the same, so we added a check inside effort_display.cpp to tell the user that he's not sending effort information coherent with joint names.