libnv: verify that string is null terminated

During unpacking, we ensure that we do not read beyond the
declared size. However, unpack uses a function that copies
null-terminated strings. Prior to this commit, if the last string
was not null-terminated, it could result in copying data into a
buffer smaller than the allocated size.

Security: FreeBSD-24:09.libnv
Security: CVE-2024-45288
Security: CAP-03
Reported by: Synacktiv
Sponsored by: The Alpha-Omega Project
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46138