Fix regression in openssl req -x509 behaviour.

Allow conversion of existing requests to certificates again.
Fixes the issue #3396

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3437)
(cherry picked from commit 888adbe0)