Crash when creating a scenario in the map editor

[CelticMinstrel]

When I create a scenario in the map editor, the game crashes in an attempt to access the AI manager.

It only happens if I've already entered a game in the current session; if I start up the game, go straight to the editor, and create a scenario, there's no crash.

[Vultraz]

This is almost certainly, since ai::manager::singleton is never reset to nullptr once the AI manager is destroyed. So, game starts, manager created, game ends, manager destroyed, but the singleton is not reset to ai::manager::has_manager() returns true when team_data is initialized for a new side in the editor.

Assigning to @jyrkive since it was his refactor originally.