pbrd: Set next/prev to NULL on cached nexthops

pbrd: Set next/prev to NULL on cached nexthops

When we copy a new nexthop to cache and track, set its
next and prev pointers to NULL. We don't want those since
this is to be treated as a single nexthop.Other nexthops that
aren't in a group could hash to this nexthop so it doesn't
make sense to keep those pointers in the cache.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>