RangeError: invalid array length while using an external source for terrain tiles

I'm experiencing the following error in both Firefox 80.0.1 on Ubuntu 18.04 and Brave browser 1.14.84:

```
An error occurred while rendering. Rendering has stopped.

RangeError: invalid array length

updateFrustums@http://localhost:1338/Build/Cesium/Cesium.js:1:2877049
View.prototype.createPotentiallyVisibleSet@http://localhost:1338/Build/Cesium/Cesium.js:1:2879945
executeCommandsInViewport@http://localhost:1338/Build/Cesium/Cesium.js:1:3063988
Scene.prototype.updateAndExecuteCommands@http://localhost:1338/Build/Cesium/Cesium.js:1:3061280
render@http://localhost:1338/Build/Cesium/Cesium.js:1:3073789
tryAndCatchError@http://localhost:1338/Build/Cesium/Cesium.js:1:3074027
Scene.prototype.render@http://localhost:1338/Build/Cesium/Cesium.js:1:3075886
CesiumWidget.prototype.render@http://localhost:1338/Build/Cesium/Cesium.js:1:3323706
e@http://localhost:1338/Build/Cesium/Cesium.js:1:3309560
FrameRequestCallback*requestAnimationFramePolyFill@http://localhost:1338/Build/Cesium/Cesium.js:1:810846
e@http://localhost:1338/Build/Cesium/Cesium.js:1:3309570
FrameRequestCallback*requestAnimationFramePolyFill@http://localhost:1338/Build/Cesium/Cesium.js:1:810846
e@http://localhost:1338/Build/Cesium/Cesium.js:1:3309570
FrameRequestCallback*requestAnimationFramePolyFill@http://localhost:1338/Build/Cesium/Cesium.js:1:810846
e@http://localhost:1338/Build/Cesium/Cesium.js:1:3309570
FrameRequestCallback*requestAnimationFramePolyFill@http://localhost:1338/Build/Cesium/Cesium.js:1:810846
e@http://localhost:1338/Build/Cesium/Cesium.js:1:3309570
FrameRequestCallback*requestAnimationFramePolyFill@http://localhost:1338/Build/Cesium/Cesium.js:1:810846
(...)
```

This error only happens when I simply move around close to the terrain (shift or rotation).

I tried with a minimalist example, using only a simple viewer. This allowed my to notice that this bug only happens when I'm using an external [terrain source](https://3d.geo.admin.ch/1.0.0/ch.swisstopo.terrain.3d/default/20200520/4326/layer.json), as documented here; https://api3.geo.admin.ch/services/sdiservices.html

I'm using it like this:
```
terrainProvider: new Cesium.CesiumTerrainProvider({
url : '//3d.geo.admin.ch/1.0.0/ch.swisstopo.terrain.3d/default/20200520/4326/'
})
```
Rights are granted. The terrain seems to load perfectly. No errors show in the console during the loading time. The crash only appears when I start moving.

Thanks for your help.

Reproduce the crash with a minimalist viewer using CesiumTerrainProvider and the linked external terrain URL, then inspect the updateFrustums and View.prototype.createPotentiallyVisibleSet stack entries in Build/Cesium/Cesium.js. Done means moving or rotating near the terrain no longer stops rendering with a RangeError: invalid array length.

Written by the indexing model from the issue text.