WallGeometry handling positions incorrectly

Reported by @primozs

`WallGeometry` is not iterating through these positions correctly, resulting in trying to normalize the zero vector:

``` js
var viewer = new Cesium.Viewer('cesiumContainer');

var dataFail = [{"x":4275839.559924875,"y":1108128.491490115,"z":4587218.256350457},{"x":4275839.560187584,"y":1108128.4915581993,"z":4587218.255867044},{"x":4275839.560187584,"y":1108128.4915581993,"z":4587218.255867044},{"x":4275839.560187584,"y":1108128.4915581993,"z":4587218.255867044},{"x":4275839.560187584,"y":1108128.4915581993,"z":4587218.255867044},{"x":4275839.560187584,"y":1108128.4915581993,"z":4587218.255867044},{"x":4275839.559983776,"y":1108128.4926396976,"z":4587218.25594444}];

var wall = {
wall: {
positions: dataFail,
material: Cesium.Color.fromCssColorString('red').withAlpha(0.2),
outline: true,
outlineColor: Cesium.Color.fromCssColorString('red').withAlpha(0.2)
}
};

viewer.entities.add(wall);

viewer.scene.camera.flyTo({
destination: new Cesium.Cartesian3(4275945.9824068565, 1107800.9025642173, 4587695.880722421),
});
```

Start by tracing WallGeometry's handling of the wall positions in the supplied viewer reproduction. Verify the repeated positions and the zero-vector normalization failure, then confirm the issue is resolved when the example renders without that error.

Written by the indexing model from the issue text.