Fix a bug in r266857: nd6_dad_find() must return NULL if it doesn't find
a matching element in the DAD queue.

Reported by:	Holger Hans Peter Freyther <holger@freyther.de>
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275695 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f