[OPENPROMIO]: Handle current_node being NULL correctly.
If the user tries to traverse to the next node of the last node, we get NULL in current_node and a zero phandle returned. That's fine, but if the user tries to obtain properties in that state, we try to dereference a NULL pointer in the downcall to the of_*() routines. So protect against that. Signed-off-by: David S. Miller <firstname.lastname@example.org>