Stat.leaveGame objective increments by 2 when the host of a LAN server logs off

If a second player is logged on, they will see it increment by only 1 at first, but when the world is entered, it will have gone up by 2.

Note: I only have one computer to test this with, it's possible that this bug doesn't occur with multiple computers. If someone could verify that please, I would appreciate it.

Relates to MC-76595.