Protect Cesium from crashing when the camera goes through terrain

[emackey]

This fixes #6783, at least for the exact error shown in the first stack trace posted on that issue.

I found an (internal) CZML that was reproducing that quite reliably, and was able to do some testing to come up with this fix.

[cesium-concierge]

Thanks for the pull request @emackey!

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Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[lilleyse]

I can't seem to reproduce the crash in master right now, but the fix looks good.