bgpd: bgp_scan shouldn't queue up route_nodes with no routes for processing

bgpd: bgp_scan shouldn't queue up route_nodes with no routes for processing

* bgp_nexthop.c: (bgp_scan) There is little point queueing an rn with no routing
  information for processing.
* bgp_route.c: (bgp_process) Do nothing on rn's with no routes. Add an assert
  for now, to try catch any other cases, but prob should be removed.
  (bgp_best_selection) rn with no routes == finish early.

(cherry picked from commit 91b9e8547a7c5697d5d7481f9476778077024019)

Conflicts:
	bgpd/bgp_nexthop.c
	bgpd/bgp_route.c