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