Hardening of several teamplay functions.

I've made several teamplay functions check their arguments more carefully. This should fix a server crash in master.

• Added Team_IsValidTeam and Team_IsValidNumber.
• IsTeamSmallerThanTeam and IsTeamEqualToTeam will now crash if given invalid teams. This should make bugs obvious.
• Removed only_return_best and return value from JoinBestTeam. They were unused and made the code much more complicated.
• AutoBalanceBots will now correctly check for given teams and issue a warning if they are invalid.