bgpd: fix "show bgp statistics" for the VPN safi

bgpd: fix "show bgp statistics" for the VPN safi

In order to iterate over MPLS VPN routes, it's necessary to use
two nested loops (the outer loop iterates over the MPLS VPN RDs,
and the inner loop iterates over the VPN routes from that RD).

The bgp_table_stats_walker() function wasn't giving this special
treatment to the MPLS VPN safi as it should, which was leading to
crashes and malfunctioning. Fix this.

Signed-off-by: Renato Westphal <renatowestphal@gmail.com>