Fix a potentially lax hash comparison

Summary: Via HackerOne. See D14025. I missed this comparison when making the original change.

Test Plan:
  - Used `cat mail.txt | scripts/mail/mail_handler.php --process-duplicates` to pipe mail in a whole lot of times.
  - Tried bad hashes, saw rejections.
  - Tried good hash, saw mail accepted.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14455

src/applications/metamta/receiver/PhabricatorObjectMailReceiver.php

@@ -126,7 +126,7 @@ abstract class PhabricatorObjectMailReceiver extends PhabricatorMailReceiver {
 
     $expect_hash = self::computeMailHash($object->getMailKey(), $check_phid);
 
-    if ($expect_hash != $parts['hash']) {
+    if (!phutil_hashes_are_identical($expect_hash, $parts['hash'])) {
       throw new PhabricatorMetaMTAReceivedMailProcessingException(
         MetaMTAReceivedMailStatus::STATUS_HASH_MISMATCH,
         pht(