Flag may attempt to decrement flag_counts below 0, causing PDO error (while deleting accounts)

I don't think deleting accounts is "impossible" in all situations, but it does reveal a bug in our incrementing/decrementing of counts. You can solve this problem in the short term by simply emptying the flag_content and flag_counts tables for whatever flag is causing the problem (looks like FID 2).

DELETE FROM flag_content WHERE fid = 2;
DELETE FROM flag_counts WHERE fid = 2;

Note that this problem does not always occur. I can delete and add accounts just fine. The reason you're running into this problem is because Flag is attempting to decrement a count below "0", which isn't allowed by our database schema. I'm not sure what could cause this situation, but we should prevent Flag from attempting to do such a SQL query in the first place.

Removing rows from the DB each time when error appears is not solution. Created patch.

We already cover this possibility when an object is unflagged, BTW:

  */
  function _decrease_count($content_id, $number = 1) {
    // Delete rows with count 0, for data consistency and space-saving.
    // Done before the db_update() to prevent out-of-bounds errors on "count".

Committed with code style tweaks.

Issue #1362260 by jaanhoinatski: Fixed user account deletion potentially causing out of bounds PDO error when decrementing flag counts.

Probably needs a backport, but patch will need considerable work to apply on 6-2.

*sigh* This is now holding up the 6-2 beta 8.

Can anyone give it a quick review?

Looks good to me. Tests clean.

Thanks! Committed.

Applies cleanly to 6-1 too.