title: view.calculateExtent returning [NaN,NaN,NaN,NaN]
body: <p>I am using OpenLayers 3.18.2.
  My map is olMap and my view is olView.
  I want to get the extent of the visible map on the screen.</p>
  
  <pre><code>olView.getCenter() = [2142126.3712395,-4085536.2324793]
  olView.getZoom() = 16
  olView.getResolution() = 2.388657133911758
  olMap.getSize() = [1906,904]
  olView.calculateExtent(olMap.getSize()) = [NaN,NaN,NaN,NaN]
  </code></pre>
  
  <p>Why is this not returning a proper extent?</p>