[Firefox] TypeError: svg is undefined TileLayer.GeoJSON.js:123

SVG root element is apparently not correctly created in Firefox.
Add preventive check and then create element if missing.
good catch!

can we update this to something like:

```
if (!this._map._pathRoot) {
     this._map._pathRoot = L.Path.prototype._createElement('svg');
     this._map._panes.overlayPane.appendChild(this._map._pathRoot);
}
var svg = this._map._pathRoot;
```