crypto: Advance the correct pointer in crypto_cursor_copydata()

PR: 271766
Reported by: Michael Laß <bevan@bi-co.net>
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40468

(cherry picked from commit 9f7fdd8c1ab153104275e59b49b2d567cec95256)