Assert that the ChessEngine has been stopped before it is destroyed

https://bugzilla.gnome.org/show_bug.cgi?id=724504

src/chess-engine.vala

@@ -58,6 +58,11 @@ public abstract class ChessEngine : Object
         this.delay_seconds = delay_seconds;
     }
 
+    ~ChessEngine ()
+    {
+        assert(!started);
+    }
+
     public bool start ()
         requires (pid == 0)
         requires (child_watch_id == 0)