Reorder requires conditions on engine_stopped_cb

Check started first and then pid as engine being started
is the primary condition post which the pid should be enforced.
https://bugzilla.gnome.org/show_bug.cgi?id=724504

src/chess-engine.vala

@@ -106,8 +106,8 @@ public abstract class ChessEngine : Object
     }
 
     private void engine_stopped_cb (Pid pid)
-        requires (pid == this.pid)
         requires (started)
+        requires (pid == this.pid)
     {
         stop (false);
         stopped_unexpectedly ();