Prevent crash caused by empty array when rendering Pholio mocks

Summary: This was causing a crash according to a conversation with @rfergu in IRC.  The issue is that if `$images_to_show` is empty, then `$image` never gets set, and it attempts to call `getID` on a null / unset variable.

Test Plan: @rfergu confirmed this fixed the crash.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, rfergu, epriestley

Differential Revision: https://secure.phabricator.com/D10073

src/applications/pholio/view/PholioMockEmbedView.php

@@ -26,6 +26,9 @@ final class PholioMockEmbedView extends AphrontView {
     if (!empty($this->images)) {
       $images_to_show = array_intersect_key(
         $this->mock->getImages(), array_flip($this->images));
+    }
+
+    if ($images_to_show) {
       foreach ($images_to_show as $image) {
         $thumbfile = $image->getFile();
         $thumbnail = $thumbfile->getThumb280x210URI();