Fixes https://github.com/nixops4/nixops4/issues/65,
possible undefined behavior.
This doesn't make the code nice wrt *const/*mut distinction, but
since we're not mutating it, this should be fine.
(cherry picked from commit 75d448aad923a5f835f0562400e223df43103ea4)
This is slightly easier to use than the previous pattern that was
always followed up by the same conversions.
(cherry picked from commit 756c080730cd4fa81d4c0e3a99688cbe8debe57f)
