Don't throw exception in diff versus file removed from revision

Test Plan: Display a revision where a file was removed compared to previous
diffs, like https://secure.phabricator.com/D1724?vs=3008&id=3009.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1754

src/applications/differential/controller/changesetview/DifferentialChangesetViewController.php

@@ -130,7 +130,7 @@ class DifferentialChangesetViewController extends DifferentialController {
     }
 
     $coverage = null;
-    if ($right->getDiffID()) {
+    if ($right && $right->getDiffID()) {
       $unit = id(new DifferentialDiffProperty())->loadOneWhere(
         'diffID = %d AND name = %s',
         $right->getDiffID(),