Fix one more "Reviewers" wire format issue

Summary: Fixes T11010. This also needs to be inflated until we fix the whole client/server responsibility issue here.

Test Plan:
  - Created a revision while observing error log, no error.
  - Disabled "allow self accept", tried to make myself a reviewer, got rejected with an error message.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11010

Differential Revision: https://secure.phabricator.com/D15966

src/applications/differential/customfield/DifferentialReviewersField.php

@@ -242,6 +242,7 @@ final class DifferentialReviewersField
     $config_self_accept_key = 'differential.allow-self-accept';
     $allow_self_accept = PhabricatorEnv::getEnvConfig($config_self_accept_key);
 
+    $value = $this->inflateReviewers($value);
     foreach ($value as $spec) {
       $phid = $spec['phid'];