Releasing all on-hold PM exceeds the allowed quota

Hi Guys,

I'm not sure if it's a bug or not. But it seems to be one.
If the number of on-hold PM is upper than the number allowed and if the Inbox is empty, when we release all PM, we can exceeds the maximum allowed in the ACP

How to quickly reproduce this bug:
ACP settings:

• Maximum private messages per box => 2

1. Empty User's PM Inboxes
2. From User_1 send a PM to User_2
3. Connect to User_2 account and check your PM
   At this point you have 1 PM in your inbox.
4. Go back to the User_1 account and send 3 PMs to User_2
5. Go back to the User_2 account and try to release your on-hold PM

After all the previous steps, User_2 Indox indicates that the folder is 50% full, but 3 PMs are on-hold. On-hold PMs can not be released unless the inbox is empty. And after the Inbox is emptied and PM are released, the Inbox indicates that the folder is 150% full.
So Inbox exceeds the allowed quota.