*: fix a bunch of segfaults detected by a CLI fuzzer

*: fix a bunch of segfaults detected by a CLI fuzzer

This patch fixes the following segfaults:
zebra aborted: vtysh -c "show ipv6 mroute"
zebra aborted: vtysh -c "configure terminal" -c "debug zebra kernel msgdump"
zebra aborted: vtysh -c "configure terminal" -c "interface eth99" -c "link-params" -c "no packet-loss"
zebra aborted: vtysh -c "configure terminal" -c "interface eth99" -c "link-params" -c "no metric"
zebra aborted: vtysh -c "configure terminal" -c "interface eth99" -c "link-params" -c "no ava-bw"
zebra aborted: vtysh -c "configure terminal" -c "interface eth99" -c "link-params" -c "no res-bw"
zebra aborted: vtysh -c "configure terminal" -c "interface eth99" -c "link-params" -c "no use-bw"
ospfd aborted: vtysh -c "configure terminal" -c "router ospf" -c "area 1.1.1.1 nssa"
ospfd aborted: vtysh -c "configure terminal" -c "router ospf" -c "area 4294967295 nssa"
pimd aborted: vtysh -c "show ip msdp sa 1.1.1.1"
pimd aborted: vtysh -c "configure terminal" -c "ip ssmpingd"
pimd aborted: vtysh -c "configure terminal" -c "no ip ssmpingd"
pimd aborted: vtysh -c "configure terminal" -c "no ip msdp mesh-group WORD source"
pimd aborted: vtysh -c "configure terminal" -c "interface eth99" -c "ip pim hello 180"
bgpd aborted: vtysh -c "show bgp l2vpn evpn rd 1:1"
bgpd aborted: vtysh -c "clear vnc nve un *"
bgpd aborted: vtysh -c "clear vnc nve un 1.1.1.1"
bgpd aborted: vtysh -c "clear vnc nve un 2001:db8::1"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 1.1.1.1 un 1.1.1.1 lifetime 4294967295"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 1.1.1.1 un 1.1.1.1 cost 255 lifetime 4294967295"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 1.1.1.1 un 1.1.1.1 cost 255"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 1.1.1.1 un 1.1.1.1"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 1.1.1.1 un 2001:db8::1 lifetime 4294967295"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 1.1.1.1 un 2001:db8::1 cost 255 lifetime 4294967295"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 1.1.1.1 un 2001:db8::1 cost 255"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 1.1.1.1 un 2001:db8::1"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 2001:db8::1 un 1.1.1.1 lifetime 4294967295"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 2001:db8::1 un 1.1.1.1 cost 255 lifetime 4294967295"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 2001:db8::1 un 1.1.1.1 cost 255"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 2001:db8::1 un 1.1.1.1"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 2001:db8::1 un 2001:db8::1 lifetime 4294967295"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 2001:db8::1 un 2001:db8::1 cost 255 lifetime 4294967295"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 2001:db8::1 un 2001:db8::1 cost 255"
bgpd aborted: vtysh -c "add vnc mac 11:11:11:11:11:11 virtual-network-identifier 4294967295 vn 2001:db8::1 un 2001:db8::1"
bgpd aborted: vtysh -c "configure terminal" -c "router bgp 1" -c "no vnc export bgp ipv4 prefix-list NAME"
bgpd aborted: vtysh -c "configure terminal" -c "router bgp 1" -c "no vnc export bgp ipv6 prefix-list NAME"
bgpd aborted: vtysh -c "configure terminal" -c "router bgp 1" -c "no vnc export bgp route-map NAME"
bgpd aborted: vtysh -c "configure terminal" -c "router bgp 1" -c "no vnc export zebra ipv4 prefix-list NAME"
bgpd aborted: vtysh -c "configure terminal" -c "router bgp 1" -c "no vnc export zebra ipv6 prefix-list NAME"
bgpd aborted: vtysh -c "configure terminal" -c "router bgp 1" -c "no vnc export zebra route-map NAME"
bgpd aborted: vtysh -c "configure terminal" -c "route-map RMAP permit 1" -c "no set ipv6 vpn next-hop"
bgpd aborted: vtysh -c "configure terminal" -c "route-map RMAP permit 1" -c "set vpnv4 next-hop"
bgpd aborted: vtysh -c "configure terminal" -c "route-map RMAP permit 1" -c "set vpnv6 next-hop"
bgpd aborted: vtysh -c "configure terminal" -c "router bgp" -c "address-family ipv6 vpn" -c "network 2001:db8::1/128 rd 1:1 tag WORD"
bgpd aborted: vtysh -c "configure terminal" -c "router bgp" -c "address-family vpnv6 unicast" -c "network 2001:db8::1/128 rd 1:1 tag WORD"
bgpd aborted: vtysh -c "configure terminal" -c "router bgp" -c "no neighbor WORD shutdown message MSG..."
bgpd aborted: vtysh -c "configure terminal" -c "router bgp" -c "neighbor WORD shutdown message MSG..."

More to come later.

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