Fix a bug in the imageproxy controller

Summary: Somehow this got through last week :( It's a bug that causes the controller to... *ahem*... just not work. Luckily nothing uses this yet so nothing was really affected.

Test Plan: Hit `/file/imageproxy/?uri=http://i.imgur.com/nTvVrYN.jpg` and are served a nice picture of a bird

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, yelirekim

Differential Revision: https://secure.phabricator.com/D16598

src/applications/files/controller/PhabricatorFileImageProxyController.php

@@ -99,7 +99,7 @@ final class PhabricatorFileImageProxyController
         ->setViewer(PhabricatorUser::getOmnipotentUser())
         ->withPHIDs(array($request->getFilePHID()))
         ->executeOne();
-      if (!file) {
+      if (!$file) {
         throw new Exception(pht(
           'The underlying file does not exist, but the cached request was '.
           'successful. This likely means the file record was manually deleted '.