bfdd: do not allow multihop peers without a local-address

bfdd: do not allow multihop peers without a local-address

The BFD code assumes that multihop peers have a local address
configured. When that doesn't happen, the BFD client daemons fail to
decode some BFD ZAPI messages and abort. To fix this, do not accept the
configuration of multhop peers unless a local-address is configured.

Fixes the following segfaults:
- bgpd aborted: vtysh -c "configure terminal" -c "bfd" -c "peer 1.1.1.1 multihop"
- bgpd aborted: vtysh -c "configure terminal" -c "bfd" -c "peer 1.1.1.1 multihop vrf NAME"
- bgpd aborted: vtysh -c "configure terminal" -c "bfd" -c "peer 1.1.1.1 vrf NAME multihop"
- ospf6d aborted: vtysh -c "configure terminal" -c "bfd" -c "peer 1.1.1.1 multihop"
- ospf6d aborted: vtysh -c "configure terminal" -c "bfd" -c "peer 1.1.1.1 multihop vrf NAME"
- ospf6d aborted: vtysh -c "configure terminal" -c "bfd" -c "peer 1.1.1.1 vrf NAME multihop"

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