PHP error with Computed field
Try to change the node_save($node) to drupal_register_shutdown_function('node_save', $node). It will remove the old node saving process from the actual node saving. 
But I don't think, that type of code should have place within a computed field. The computed field good to modify the same field. It isn't designed for modifying other data of your system.
These code should be in a custom module in hook_node_presave().