Fix issue reported from github

Summary:
we filter the $actors above such that its possible to have no $actor anymore (if $actor is not a deliverable email address). ergo, make sure we have actor before we start calling methods.

Fixes github issue 403

Test Plan: logic on this one - not 100% sure how to easily reproduce

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7284

src/applications/metamta/storage/PhabricatorMetaMTAMail.php

@@ -383,10 +383,13 @@ final class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
         switch ($key) {
           case 'from':
             $from = $value;
-            $actor = $actors[$from];
-
-            $actor_email = $actor->getEmailAddress();
-            $actor_name = $actor->getName();
+            $actor_email = null;
+            $actor_name = null;
+            $actor = idx($actors, $from);
+            if ($actor) {
+              $actor_email = $actor->getEmailAddress();
+              $actor_name = $actor->getName();
+            }
             $can_send_as_user = $actor_email &&
               PhabricatorEnv::getEnvConfig('metamta.can-send-as-user');