Fix nullpointer dereference in OSSL_PARAM_merge

Fix nullpointer dereference in OSSL_PARAM_merge

OSSL_PARAM_merge contained an error, where a nullpointer was
dereferenced when both parameter arrays ended with the same key

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27910)

(cherry picked from commit 63cb8f99a13fdc4c7c3b1e88d66a3ff70b72e642)