bgpd: vpn null check

bgpd: vpn null check

Can be reproduced with following vtysh sequence (an intentionaly wrong
sequence):

['configure terminal\n router bgp 255 vrf l3vrf-6\n',
 'configure terminal\n router bgp 255 vrf l3vrf-6\nbgp router-id 10.255.255.1\n',
 'configure terminal\n router bgp 255 vrf l3vrf-6\n address-family ipv4\nlabel vpn export 45000\n',
 'configure terminal\n router bgp 255 vrf l3vrf-6\n address-family ipv4\nrd vpn export 255:5\n',
 'configure terminal\n router bgp 255 vrf l3vrf-6\n address-family ipv4\nredistribute isis\n', 'configure terminal\n router bgp 255 vrf l3vrf-6\n address-family ipv4\nredistribute ospf\n', 'configure terminal\n router bgp 255 vrf l3vrf-6\n address-family ipv4\nredistribute connected\n', 'configure terminal\n router bgp 255 vrf l3vrf-6\n address-family ipv4\nimport vpn\n', 'configure terminal\n router bgp 255 vrf l3vrf-6\n address-family ipv4\nexport vpn\n', 'configure terminal\n router bgp 255 vrf l3vrf-6\n address-family ipv4\nrt vpn import 255:1\n', 'configure terminal\n router bgp 255 vrf l3vrf-6\n address-family ipv4\nrt vpn export 255:1\n'
]

Signed-off-by: F. Aragon <paco@voltanet.io>