Fix error in view-padding example

An error occured when the buttons are clicked before the source is loaded.

examples/view-padding.js

@@ -50,6 +50,7 @@ const map = new Map({
   view: view,
 });
 
+vectorLayer.getSource().on('featuresloadend', function () {
   const zoomtoswitzerland = document.getElementById('zoomtoswitzerland');
   zoomtoswitzerland.addEventListener(
     'click',
@@ -71,3 +72,4 @@ centerlausanne.addEventListener(
     },
     false
   );
+});