bgpd: BGP fails to free the nexthop node

bgpd: BGP fails to free the nexthop node

In case of BGP unnumbered, BGP fails to free the nexthop
node for peer if the interface is shutdown before
unconfiguring/deleting the BGP neighbor.

This is because, when the interface is shutdown,
peer's LL neighbor address will be cleared. Therefore,
during neighbor deletion, since the peer's neighbor
address is not available, BGP will skip freeing the
nexthop node of this peer. This results in a stale
nexthop node that points to a peer that's already
been freed.

Ticket: 3191547
Signed-off-by: Pooja Jagadeesh Doijode <pdoijode@nvidia.com>