Fix fatal on setting default profile picture

Summary: I thought I fixed this but must have not saved in my editor or something. Fixes T3552.

Test Plan: Set profile image to default.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3552

Differential Revision: https://secure.phabricator.com/D6477

src/applications/people/controller/PhabricatorPeopleProfilePictureController.php

@@ -78,7 +78,11 @@ final class PhabricatorPeopleProfilePictureController
       }
 
       if (!$errors) {
-        $user->setProfileImagePHID($xformed->getPHID());
+        if ($is_default) {
+          $user->setProfileImagePHID(null);
+        } else {
+          $user->setProfileImagePHID($xformed->getPHID());
+        }
         $user->save();
         return id(new AphrontRedirectResponse())->setURI($profile_uri);
       }