Wrong count of non-read PMs

It seems that going to Private messages page always resets the non-read messages count, instead of counting the number of red flags in front of messages

Steps to reproduce:

• read all your PM (non-read count is 0, ie 'Private messages' is in black)
• ask someone to send you a PM
• check that 'Private messages' changed to red bold, with 'Privates messages (1)' text
• click on it, but don't read the new PM
• go back to forum menu
• check that 'Private messages' in now black, even if you don't have read the message