Fix store with opaque data

When experimental-store is enabled, it does not compile due to the
change to opaque data structures.

Change CRYPTO_add() to EVP_PKEY_up_ref() as needed.

Signed-off-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Rich Salz <rsalz@openssl.org>

RT: #4263, GH: #579