bgpd: do not unregister for prefix nexthop updates if nh exists

bgpd: do not unregister for prefix nexthop updates if nh exists

since the addition of srte_color to the comparison for bgp nexthops
it is possible to have several nexthops per prefix but since zebra
only sores a per prefix registration we should not unregister for
nh notifications for a prefix unti all the nexthops for that prefix
have been deleted. Otherwise we can get into a deadlock situation
where BGP thinks we have registered but we have unregistered from zebra.

Signed-off-by: Pat Ruddy <pat@voltanet.io>