Assertion failure during fuzzing

[jonathanolson]

Error: Assertion failed: A rectangle needs to have a non-negative finite height (NaN)
    at window.assertions.assertFunction (http://localhost/phet/git/assert/js/assert.js:22:13)
    at Rectangle.inherit.invalidateRectangle (http://localhost/phet/git/scenery/js/nodes/Rectangle.js?bust=1423091348093:280:87)
    at Rectangle.(anonymous function) [as setRectHeight] (http://localhost/phet/git/scenery/js/nodes/Rectangle.js?bust=1423091348093:428:14)
    at SingleEnergyGraphNode.inherit.updateEnergy (http://localhost/phet/git/pendulum-lab/js/energy/view/SingleEnergyGraphNode.js?bust=1423091348093:140:12)
    at SingleEnergyGraphNode.inherit.updatePotentialEnergy (http://localhost/phet/git/pendulum-lab/js/energy/view/SingleEnergyGraphNode.js?bust=1423091348093:150:14)
    at Property.inherit._notifyObservers (http://localhost/phet/git/axon/js/Property.js?bust=1423091348093:103:29)
    at Property.inherit._setAndNotifyObservers (http://localhost/phet/git/axon/js/Property.js?bust=1423091348093:89:14)
    at Property.inherit.set (http://localhost/phet/git/axon/js/Property.js?bust=1423091348093:62:16)
    at Pendulum.Object.defineProperty.set [as potentialEnergy] (http://localhost/phet/git/axon/js/PropertySet.js?bust=1423091348093:133:43)
    at Pendulum.inherit.updateEnergiesWithThermalEnergyConstant (http://localhost/phet/git/pendulum-lab/js/common/model/Pendulum.js?bust=1423091348093:146:28)

To launch to reproduce this error, use ?ea&fuzzMouse=100. If you have troubles reproducing, let me know.

[andrey-zelenkov]

Fixed assertion failure during fuzzing. Prevent undefined gravity value. Reassign to @jonathanolson for verification.

[jonathanolson]

Looks good, thanks!