Fix logic in network_dump_upcall()

Fix logic in network_dump_upcall()

The nexthop can be valid but still a NULL pointer. In that case just set
the aid like it is done for invalid nexthops. If the nexthop is set by
explicitly specifying one then include the exit_nexthop, that is the
nexthop that is relevant for BGP. Further cleanup the function as usual.
OK tb@