Silence an undeclared variable warning

Summary:
If you don't have a custom renderer, this variable does not get set
and emits a warning when you try to read it.

Test Plan:
Loaded page before and after change, warnings went away (lines
145 and 154 in old file).

Reviewed By: aran
Reviewers: tuomaspelkonen, aran
CC: jungejason, aran, epriestley
Differential Revision: 186

src/applications/differential/controller/revisionview/DifferentialRevisionViewController.php

@@ -135,6 +135,8 @@ class DifferentialRevisionViewController extends DifferentialController {
       PhutilSymbolLoader::loadClass($custom_renderer_class);
       $custom_renderer =
         newv($custom_renderer_class, array());
+    } else {
+      $custom_renderer = null;
     }
 
     $properties = $this->getRevisionProperties(