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