lib: assert if someone adds to nexthop list to nhg

lib: assert if someone adds to nexthop list to nhg

If someone tries to add a nexthop with a list of nexthops
already attached to it, let's just assert. This standardizes
the API to say we assume this is an individual nexthop
you are appending to a group.

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