Fix #11585: Crash when cleaning AI/GS with nested AsyncMode.

Do not throw sanity check when in_shutdown is true. This is also applied to
ExecMode and TestMode as they follow the same pattern.

src/script/script_instance.hpp

@@ -152,6 +152,11 @@ public:
 	 */
 	inline bool IsDead() const { return this->is_dead; }
 
+	/**
+	 * Return whether the script is alive.
+	 */
+	inline bool IsAlive() const { return !this->IsDead() && !this->in_shutdown; }
+
 	/**
 	 * Call the script Save function and save all data in the savegame.
 	 */