evp: detect and raise an error if no digest is found for a sign/verify operation

evp: detect and raise an error if no digest is found for a sign/verify operation

If no digest is specified, the code looks for a default digest per PKEY via the
evp_keymgmt_util_get_deflt_digest_name() call.  If this call returns NULL,
indicating no digest found, the code continues regardless.  If the verify/sign
init later fails, it returns an error without raising one.  This change raises
an error in this case.

Fixes #15372

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16015)