fix: Add a guard if a vgl geometry asks for indices before it is set

Also, reenable a test that was disabled ages ago.

Author: manthey

src/vgl/geomData.js

@@ -35,7 +35,7 @@ vgl.primitive = function () {
    * @returns {number} The number of indices.
    */
   this.numberOfIndices = function () {
-    return m_indices.length;
+    return m_indices === null ? 0 : m_indices.length;
   };
 
   /*

tests/cases/sliderWidget.js

@@ -1,7 +1,7 @@
 var geo = require('../test-utils').geo;
 var createMap = require('../test-utils').createMap;
 
-xdescribe('zoom slider', function () {
+describe('zoom slider', function () {
   'use strict';
 
   var d3 = require('d3');