crypto/engine/eng_devcrypto.c: add digest copy

Copying an EVP_MD_CTX, including the implementation local bits, is a
necessary operation.  In this case, though, it's the same as
initializing the local bits to be "copied to".

Fixes #7495

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/7506)

(cherry picked from commit 36af124b)