bgpd: fix incorrect json in bgp_show_table_rd

bgpd: fix incorrect json in bgp_show_table_rd

In bgp_show_table_rd(), the is_last argument is determined using the
expression "next == NULL" to check if the RD table is the last one. This
helps ensure proper JSON formatting.

However, if next is not NULL but is no longer associated with a BGP
table, the JSON output becomes malformed.

Updates the condition to also verify the existence of the next bgp_dest
table.

Fixes: 1ae44dfcba ("bgpd: unify 'show bgp' with RD with normal unicast bgp show")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>