ospf6d: fix ospf6 asbr crash

ospf6d: fix ospf6 asbr crash

Durig ospf6 instance cleanup all border routers
are removed from the db then external LSAs removal
from DB is triggered. During the time, external route
path would not be valid as brouters along with its
rechability have vanished.
For a given external route removal check if no more
paths available simple remove the route from route db.

Ticket:CM-20669
Testing Done:
Bring up ASBR configuration with ECMP paths to a route.
Bring down the ospf6 instance and validate route is removed
from the DB.

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>