Default route not removed

If a node leaves the DAG, the default route is not removed correctly. Therefore the node cannot return to the network. This leads to an "unacceptable rank" error.

In *rpl-dag.c*, function *rpl_process_parent_event*:
- the function *rpl_select_dag* sets the preferred dag to *NULL* (line 637)
- *rpl_local_repair* cannot nullify the parents, because *parent == dag->preferred_parent* fails (zero pointer)
- This fix checks if *dag->preferred_parent* equals *NULL* and starts the local repair

This fix solves this problem. Tested with sky and z1 in cooja.