Fix fatal when deleted user is subscribed to a task and we generate an email to them

Summary: Fixes T3528. We won't be able to load the user if they've been deleted, and will fatal a few lines later on `$user->getID()`.

Test Plan: I'm going with my gut on this one.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3528

Differential Revision: https://secure.phabricator.com/D6442

src/applications/metamta/replyhandler/PhabricatorMailReplyHandler.php

@@ -290,6 +290,12 @@ EOBODY;
       ->withPHIDs(array($handle->getPHID()))
       ->executeOne();
 
+    if (!$user) {
+      // This may happen if a user was subscribed to something, and was then
+      // deleted.
+      return null;
+    }
+
     $receiver = $this->getMailReceiver();
     $receiver_id = $receiver->getID();
     $user_id = $user->getID();