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
MFC after: 3 days
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