Recent changes to the nht code in bgp caused us to actually
keep a true count of v6 nexthop paths when using v4 over v6.
This change introduced a race condition on shutdown on who
got to the bnc cache first( the v4 table or not ). Effectively
we were allowing the continued existence of the path->nexthop
pointing to the freed bnc. This was especially true when
we had route leaking. So when we free the bnc make sure
we clean up the path->nexthop variables pointing at it too.
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>