Fix an ENGINE leak in asn1_item_digest_with_libctx

Fix an ENGINE leak in asn1_item_digest_with_libctx

Commit 6725682d introduced a call to ENGINE_get_digest_engine() into
the function asn1_item_digest_with_libctx() to determine whether there
is an ENGINE registered to handle the specified digest. However that
function increases the ref count on the returned ENGINE object, so it
must be freed.

Fixes #12558

[extended tests]

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/12560)