"show bgp vpn rd RD json" returns in invalid JSON unless RD is the last
route-distinguisher in the table. A trailing "," is present.

> r1# sh bgp ipv4 vpn rd 102:1 json
> {
>  "vrfId": 0,
>  "vrfName": "default",
>  "tableVersion": 2,
>  "routerId": "192.0.2.1",
>  "defaultLocPrf": 100,
>  "localAS": 65001,
>  "routes": {  "routeDistinguishers" : { "102:1" : { "2.2.2.2/32": [{"valid":true,"bestpath":true,"selectionReason":"First path received","pathFrom":"external","prefix":"2.2.2.2","prefixLen":32,"network":"2.2.2.2/32","version":1,"metric":0,"weight":0,"peerId":"192.0.2.2","path":"65002","origin":"incomplete","nexthops":[{"ip":"192.0.2.2","hostname":"r2","afi":"ipv4","used":true}]
> }]
> ,"192.0.2.2/32": [{"pathFrom":"external","prefix":"192.0.2.2","prefixLen":32,"network":"192.0.2.2/32","version":0,"metric":0,"weight":0,"peerId":"192.0.2.2","path":"65002","origin":"incomple
> te","nexthops":[{"ip":"192.0.2.2","hostname":"r2","afi":"ipv4","used":true}]}]
> ,"192.0.2.8/32": [{"valid":true,"bestpath":true,"selectionReason":"First path received","pathFrom":"external","prefix":"192.0.2.8","prefixLen":32,"network":"192.0.2.8/32","version":2,"metric
> ":0,"weight":0,"peerId":"192.0.2.2","path":"65002","origin":"incomplete","nexthops":[{"ip":"192.0.2.2","hostname":"r2","afi":"ipv4","used":true}]}]
>  }, r1#

When there is another rd, "next" is set although it refers to another rd
that won't be displayed. And "next" is used to set the "is_last"
variable of bgp_show_table() that instructs whether to add a trailing ","

Do not set next if there no more rd to display.

Fixes: 1ae44df ("bgpd: unify 'show bgp' with RD with normal unicast bgp show")
Signed-off-by: Maxime Gouin <maxime.gouin@6wind.com>
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>
(cherry picked from commit 03f782b)