ndp: cope with unresolved neighbours

If we've not (yet) resolved a neighbour nda_lladdr will be NULL, and
NLA_DATA_LEN(neigh->nda_lladdr) will dereference a NULL pointer.

Avoid that by checking nda_lladdr first, and only dereferencing if it's
not NULL.

Test case:
ping6 -c 1 <non-existant neighbour>
ndp -a

Reviewed by: melifaro
MFC after: 3 days
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D41903

(cherry picked from commit b57df6fbcc484f1941bf306cb60a3adaf538df69)