Avoid sending the map change order when selecting the same map.  

Fixes #6206.

Switching the map causes the client state to be changed to `Invalid`.  `UpdateCurrentMap` would normally verify that the map exists and then change the client state back to `NotReady`, but this check is skipped when switching to the same map.  If the game is then started with the force-start button, the server will notice that all of the players have an `Invalid` state and kick them as if they did not have the map.

Preventing the initial map order from being issued avoids this.