Fix time series google map

Check if layer exists to prevent an error.
More info [here](https://github.com/CartoDB/support/issues/1498#issuecomment-386035383)

lib/torque/renderer/point.js

@@ -142,7 +142,7 @@ var CartoDatasource = require('./datasource');
       this._shader = shader;
       this._Map = this._shader.getDefault().getStyle({}, { zoom: 0 });
       var img_names = this._shader.getImageURLs();
-      if (this.layer.showLimitErrors) {
+      if (this.layer && this.layer.showLimitErrors) {
         img_names.push(ERROR_IMG_URL);
       }