spapr_drc: abort if object_property_add_child() fails

spapr_drc: abort if object_property_add_child() fails

object_property_add_child() can only fail in two cases:
- the child already has a parent, which shouldn't happen since the DRC was
  allocated a few lines above
- the parent already has a child with the same name, which would mean the
  caller tries to create a DRC that already exists

In both case, this is a QEMU bug and we should abort.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>