Fix a possible undefined variable

Summary: Fixes T7881.

Test Plan: Inspection.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T7881

Differential Revision: https://secure.phabricator.com/D12500

src/applications/differential/controller/DifferentialChangesetViewController.php

@@ -175,6 +175,7 @@ final class DifferentialChangesetViewController extends DifferentialController {
 
     $can_mark = false;
     $object_owner_phid = null;
+    $revision = null;
     if ($revision_id) {
       $revision = id(new DifferentialRevisionQuery())
         ->setViewer($viewer)