Crash in PolylineGraphics

I have following code:

```
var pickPositions = [];
var lineStrip;
var handler = new Cesium.ScreenSpaceEventHandler(viewer.scene.canvas);
handler.setInputAction(function(movement) {
if (!viewer.scene.pickPositionSupported) {
console.warn("pickPosition not supported!");
return;
}

var pickedPosition = viewer.scene.pickPosition(movement.position);

if (pickedPosition !== undefined) {
pickPositions.push(pickedPosition);

if (pickPositions.length > 1) {
//viewer.entities.remove(lineStrip); //<- add this line here and all works fine

lineStrip = viewer.entities.add({
polyline : {
positions: pickPositions,
width : 10,
material: Cesium.Color.ROYALBLUE,
followSurface: true
}
});
}
}
}, Cesium.ScreenSpaceEventType.LEFT_DOUBLE_CLICK);
```

At the moment I picked 5. point Cesium is crashing. I'm getting following error:
```
RangeError: Invalid array length
RangeError: Invalid array length
at Function.PolylineGeometry.unpack (http://localhost:8080/Source/Core/PolylineGeometry.js:248:35)
at createPolylineGeometry (http://localhost:8080/Source/Workers/createPolylineGeometry.js:14:49)
at createGeometry (http://localhost:8080/Source/Workers/createGeometry.js:46:30)
at http://localhost:8080/Source/Workers/createTaskProcessorWorker.js:56:42
```
It works if I remove `lineStrip` each time before adding new `lineStrip`. So it looks like Cesium isn't handling changes in position array correctly (I have no idea why it crashes at 5 points, but not already at 3).

My expectation was an error message that altering position after it is already added to scene is not allowed - or if altering is supported no crash.

Reproduce the crash with the provided entity and changing pickPositions array. Start with Source/Core/PolylineGeometry.js at unpack and follow the calls through Source/Workers/createPolylineGeometry.js and createGeometry.js. Done means changing the positions after adding the polyline no longer crashes, or produces the expected error behavior.

Written by the indexing model from the issue text.