fix for persistent "you have n new private messages" message (caused by deleted users).

Well, after writing this code and thinking more about it, I think the real problem is the SQL query that generates the message list. There must be some way to rewrite it using a join so that even if the author no longer exists in the users table, the message will still show up. That would be the better way to solve this problem.

These two queries would have to be updated to LEFT JOIN queries, so that non-existent users would still produce a result, just without any user info (set to NULL).

$result = pager_query('SELECT id, subject, p.timestamp, u.uid, u.name, newmsg FROM {privatemsg} p, {users} u WHERE p.author = u.uid AND p.recipient = %d AND folder = %d AND p.recipient_del = 0 ORDER BY p.timestamp DESC', variable_get('privatemsg_per_page', 10), 0, NULL, $user->uid, $current_folder);

SHOULD BE
$result = pager_query('SELECT id, subject, p.timestamp, u.uid, u.name, newmsg FROM {privatemsg} p LEFT JOIN {users} u ON p.author = u.uid WHERE p.recipient = %d AND folder = %d AND p.recipient_del = 0 ORDER BY p.timestamp DESC', variable_get('privatemsg_per_page', 10), 0, NULL, $user->uid, $current_folder);

Rewriting the query this way will allow messages to be displayed even if the user has been deleted. The fields for uid and name will be filled with null, that's all, and that's certainly preferable to the message no showing up.

Similarly, this should be rewritten, although that's not as critical, considering it would be unlikely for a deleted user to look at their sent messages:
// sent messages
$result = pager_query('SELECT id, subject, p.timestamp, u.uid, u.name, newmsg FROM {privatemsg} p, {users} u WHERE p.recipient = u.uid AND p.author = %d AND p.author_del = 0 ORDER BY p.timestamp DESC', variable_get('privatemsg_per_page', 10), 0, NULL, $user->uid);

What do you guys think? Worth rolling a patch? I think the rewrite of the query is the way to go. The patch at top should probably still be in there, just to clean up gunk that cannot be cleaned out in any other way (a deleted user cannot delete their messages, for example).

For some reason, my patch file was deleted. Submitting again.