Tolerate 0 byte input length for Update functions

We treat that as automatic success. Other EVP_*Update functions already do
this (e.g. EVP_EncryptUpdate, EVP_DecryptUpdate etc). EVP_EncodeUpdate is
a bit of an anomoly. That treats 0 byte input length as an error.

Fixes openssl#8576

crypto/evp/digest.c

@@ -259,6 +259,9 @@ int EVP_DigestInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl)
 
 int EVP_DigestUpdate(EVP_MD_CTX *ctx, const void *data, size_t count)
 {
+    if (count == 0)
+        return 1;
+
     if (ctx->digest == NULL || ctx->digest->prov == NULL)
         goto legacy;
 

crypto/evp/mac_lib.c

@@ -82,6 +82,8 @@ int EVP_MAC_init(EVP_MAC_CTX *ctx)
 
 int EVP_MAC_update(EVP_MAC_CTX *ctx, const unsigned char *data, size_t datalen)
 {
+    if (datalen == 0)
+        return 1;
     return ctx->meth->update(ctx->data, data, datalen);
 }