Handle invalid input more gracefully

Nobody has claimed this yet.

Some of my zoom levels are returning an empty list cluster, while other zoom levels are properly returning a list of clusters.


```javascript

const index = new Supercluster({
radius: 200,
maxZoom: 16,
});
index.load(markers);
const clusterResponse = {};

const ZOOM_LEVELS = [9, 10, 11, 12, 13, 14, 15];
ZOOM_LEVELS.forEach((zoomLevel) => {
const clusters = index.getClusters([-180, -85, 180, 85], zoomLevel);
clusterResponse[zoomLevel.toString()] = clusters
.map(cluster => ({
id: cluster.id || cluster.properties.id,
count: cluster.properties.cluster ? cluster.properties.point_count : 1,
longitude: cluster.geometry.coordinates[0],
latitude: cluster.geometry.coordinates[1],
}));
});
```

Start by reproducing the example in issue #128 with the supplied markers, world-spanning bounds, and zoom levels 9–15. Inspect the getClusters entry point and compare its returned values at the failing zoom levels. Done means the reported input no longer produces an unexpected empty cluster list, with regression coverage for the case.

Written by the indexing model from the issue text.