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 #8576
Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8587)

(cherry picked from commit a8274ea351988aa754cb9983b27d7059613ee11e)

crypto/evp/digest.c

@@ -150,6 +150,9 @@ 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;
+
     return ctx->update(ctx, data, count);
 }