[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 <davem@davemloft.net>