libnv: fix double free

In r343986 we introduced a double free. The structure was already
freed fixed in the r302966. This problem was introduced
because the GitHub version was out of sync with the FreeBSD one.

Submitted by: Mindaugas Rasiukevicius <rmind@netbsd.org>
MFC with: r343986

(cherry picked from commit d97753b5c8f1d32fbcdcbb0d129b49f808245865)