Fix PolylineCollection.destroy

Originally reported on the mailing list: https://groups.google.com/d/msg/cesium-dev/gHnPxShEWTY/0du0hPm0fVEJ It turns out that our specs were never destroying the PolylineCollection, which was hiding the issue.  I updated the specs to destroy after each and also fixed the issue, which was a missing undefined check.