Fix two minor issues with Pholio

Summary:
  - One bad method call.
  - One fatal when handling a mock with no images. These "can't" exist, but we have a few old ones I think and it's probably better not to fatal. We could filter the object out completely, but this doesn't seem like a bad fix?

Test Plan: Viewed `/pholio/` (although I don't think I have any imageless mocks) and viewed an outdated image.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6574

src/applications/pholio/controller/PholioMockViewController.php

@@ -134,7 +134,7 @@ final class PholioMockViewController extends PholioController {
     $image = idx($images, $image_id);
 
     if ($image) {
-      $history = $mock->getImageUpdateSet($image_id);
+      $history = $mock->getImageHistorySet($image_id);
       $latest_image = last($history);
       $href = $this->getApplicationURI(
         'image/history/'.$latest_image->getID().'/');