window-tracker: Fix memory leak

shell_window_tracker_get_window_app() returns a new reference
which we need to drop once we're done.

https://bugzilla.gnome.org/show_bug.cgi?id=722927

src/shell-window-tracker.c

@@ -484,6 +484,8 @@ update_focus_app (ShellWindowTracker *self)
     }
 
   set_focus_app (self, new_focus_app);
+
+  g_clear_object (&new_focus_app);
 }
 
 static void