libnv: allocate buffer in a safe way

Ensure that the calculation of size of array doesn't
overflow.

Security: FreeBSD-24:09.libnv
Security: CVE-2024-45287
Security: CAP-02
Reported by: Synacktiv
Reported by: Taylor R Campbell (NetBSD)
Sponsored by: The Alpha-Omega Project
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46131

(cherry picked from commit 36fa90dbde0060aacb5677d0b113ee168e839071)
(cherry picked from commit 2e7f9244ebefe019ef016a3a5b47c4562850d1c2)

Approved by: so
Approved by: re (cperciva)