Fix: abort world generation on exiting the game as soon as possible

This prevents the window from "freezing" when you close it during
world generation, as it first would continue the action.

src/genworld.cpp

@@ -243,7 +243,7 @@ void AbortGeneratingWorld()
  */
 bool IsGeneratingWorldAborted()
 {
-	return _gw.abort;
+	return _gw.abort || _exit_game;
 }
 
 /**