Fix EC_KEY_set_private_key() priv_key regression

This allows to set EC_KEY's private key to NULL and fixes regression
issue following OTC guideline in
https://github.com/openssl/openssl/issues/18744#issuecomment-1195175696



Fixes #18744.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/18942)