title: Strange writeVideoAtPathToSavedPhotosAlbum behaviour
body: <p>We have <strong>"video-replated"</strong> app and using <code>writeVideoAtPathToSavedPhotosAlbum:completionBlock:</code> from <code>ALAssetsLibrary</code> to save videos into camera roll. Most of the time it works very well, but sometimes <code>completionBlock:^(NSURL *assetURL, NSError *error)</code> called with BOTH parameters <code>equal</code> to <code>nil</code>. </p>
  
  <p>Documentation states that in case error is nil - no error occurred during write operation, so <code>assetURL</code> should not be <code>nil</code>. But still we get <code>assetURL == nil</code> and <code>error == nil</code> at the same time, sometimes</p>
  
  <p>In what cases this is possible? Any help welcome.</p>