title: Issue with ALAssetsLibrary
body: <p>I am trying to access the last photo taken on the Photo library , everything works fine except if , photo library has no photo my app crashes ! how can I find a solution to stop crashing ? here is my code :</p>
  
  <pre><code>-(void)importLastImage {
  
      ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
  
      // Enumerate just the photos and videos group by using ALAssetsGroupSavedPhotos.
      [library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
  
          // Within the group enumeration block, filter to enumerate just photos.
          [group setAssetsFilter:[ALAssetsFilter allPhotos]];
  
          // Chooses the photo at the last index
          [group enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:([group numberOfAssets]-1)]
                                  options:0
                               usingBlock:^(ALAsset *alAsset, NSUInteger index, BOOL *innerStop) {
  
                                   // The end of the enumeration is signaled by asset == nil.
                                   if (alAsset) {
  
  
                                       ALAssetRepresentation *representation = [alAsset defaultRepresentation];
                                       latestPhoto = [UIImage imageWithCGImage:[representation fullResolutionImage]];
  
  
  
                                   }else {
  
                                   }
         }];
      }
                           failureBlock: ^(NSError *error) {
                               // Typically you should handle an error more gracefully than this.
                               NSLog(@"No groups");
  
  
  }];
  
  }
  </code></pre>