Incorrect unread count for deleted messages

Site admins and managers can delete messages not only for themselves but also for the recipients.

Once an admin or manager sends a message and then immediately deletes it for the recipient, the recipient sees an incorrect unread message count in the page header.

If the recipient has conversation with the sender, they can correct the count by opening the conversation, although it can be difficult to identify which conversation is causing the incorrect count.  If the recipient has no conversation with the sender, it is impossible to correct the count.

Reproduction Steps:

1. Log in as a site admin or manager.
2. Choose a user who has not received a message ever from you.
3. Send a message to the user.
4. Delete the message for you and for everyone else.
5. Log out.
6. Log in as the user and you see the incorrect count that you can not correct.