http_digest: Reworked the SSPI based input token storage

Reworked the input token (challenge message) storage as what is passed
to the buf and desc in the response generation are typically blobs of
data rather than strings, so this is more in keeping with other areas
of the SSPI code, such as the NTLM message functions.

lib/urldata.h

@@ -393,6 +393,7 @@ struct curl_ssl_session {
 struct digestdata {
 #if defined(USE_WINDOWS_SSPI)
   BYTE *input_token;
+  size_t input_token_len;
 #else
   char *nonce;
   char *cnonce;