EVP_DigestUpdate(): Check if ctx->update is set

EVP_DigestUpdate(): Check if ctx->update is set

The issue has been discovered by libFuzzer running on provider target.
There are currently three distinct reports which are addressed by
code change here.

    https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=69236#c1
    https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=69243#c1
    https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=69261#c1

the issue has been introduced with openssl 3.0.

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24753)