bgpd: fix segfault with some show commands

bgpd: fix segfault with some show commands

The following commands were causing bgpd to crash when vpnv4/vpnv6 static
routes are configured:

bgpd aborted: vtysh  -c "show ip bgp view all ipv4 vpn json"
bgpd aborted: vtysh  -c "show ip bgp view all ipv4 vpn"
bgpd aborted: vtysh  -c "show ip bgp view all ipv6 vpn json"
bgpd aborted: vtysh  -c "show ip bgp view all ipv6 vpn"
bgpd aborted: vtysh  -c "show ip bgp vrf all ipv4 vpn json"
bgpd aborted: vtysh  -c "show ip bgp vrf all ipv4 vpn"
bgpd aborted: vtysh  -c "show ip bgp vrf all ipv6 vpn json"
bgpd aborted: vtysh  -c "show ip bgp vrf all ipv6 vpn"
bgpd aborted: vtysh  -c "show bgp view all ipv4 vpn json"
bgpd aborted: vtysh  -c "show bgp view all ipv4 vpn"
bgpd aborted: vtysh  -c "show bgp view all ipv6 vpn json"
bgpd aborted: vtysh  -c "show bgp view all ipv6 vpn"
bgpd aborted: vtysh  -c "show bgp vrf all ipv4 vpn json"
bgpd aborted: vtysh  -c "show bgp vrf all ipv4 vpn"
bgpd aborted: vtysh  -c "show bgp vrf all ipv6 vpn json"
bgpd aborted: vtysh  -c "show bgp vrf all ipv6 vpn"

Problem found with the CLI fuzzer.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>