Fix two crashes.

1. `PolylineCollection` would try and clone an undefined BoundingSphere
2. `CustomSensorVolume` had a bad function call due to the recent "private" function work.

@bagnell is my fix for 1 correct?

See this thread: https://groups.google.com/forum/?fromgroups=#!topic/cesium-dev/iR7Z230g3lE

Should we consider a b16a?