EVP_DigestInit_ex(): drop previous context engine earlier

If a EVP_MD_CTX holds a reference to a previously given engine, and
the type of its digest isn't the same as the one given in the new
call, drop that engine reference, allowing providers or other engines
to provide the new algorithm on an equal basis.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9077)