src: fix SetClientCertEngine() nullptr dereference

Introduced in commit 6ee985f3 ("tls: implement clientCertEngine
option") which was merged November 11.

PR-URL: https://github.com/nodejs/node/pull/16965


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>