Crashing - Probably due to request for negative tiles

[MortenHofft]

To reproduce

• take any map (e.g. https://www.mapbox.com/mapbox-gl-js/examples/)
• Zoom out.
• Pan around the world to negative coordinates (e.g. Decisions #1.84/-30.0/-708.7)
• Zoom in. And an error is thrown in TileCoord that locks the map completely.

Console: Uncaught TypeError: Cannot read property 'replace' of undefined

TileCoord.prototype.url = function(urls, sourceMaxZoom) {
    return urls[(this.x + this.y) % urls.length]

(this.x + this.y) % urls.length] is a negative number.

It can also be seen in the failed request for tiles a la:

https://a.tiles.mapbox.com/v4/mapbox.mapbox-terrain-v2,mapbox.mapbox-streets-v6/1/-1/1.vector.pbf?access_token=pk.eyJ...A