Set con window to NULL on tree_close

Long description:

Prevents invalid reads on the freed window struct.

To apply this patch, use:
curl http://cr.i3wm.org/patch/455/raw.patch | git am

@@ -260,7 +260,7 @@ bool tree_close(Con *con, kill_window_t kill_window, bool dont_kill_parent, bool

         FREE(con->window->class_class);

         FREE(con->window->class_instance);

         i3string_free(con->window->name);

-        free(con->window);

+        FREE(con->window);

     }

     Con *ws = con_get_workspace(con);