Fix null pointer check in pkey_dh_derive to ensure both keys are set

Fix null pointer check in pkey_dh_derive to ensure both keys are set

CLA: trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/28259)

(cherry picked from commit fc84d46d7227886152be00618889a521e9132ef3)
(cherry picked from commit 0163c6ad1f7cec08f59e9f736d36073d01ae7ae5)
(cherry picked from commit 257c3dd6cdbe9b1bde353fa45445b1319de6ffc3)