ssh-agent: promote sk_provider == NULL to "internal"

a sk_provider is required by ssh-sk-helper. as such, treat ssh-sk
keys without a provider as belonging to the "internal" provider.

contrib/win32/win32compat/ssh-agent/keyagent-request.c

@@ -208,6 +208,7 @@ static int sign_blob(const struct sshkey *pubkey, u_char ** sig, size_t *siglen,
 	DWORD regdatalen = 0, keyblob_len = 0;
 	struct sshbuf* tmpbuf = NULL;
 	char *keyblob = NULL;
+	const char *sk_provider = NULL;
 
 	*sig = NULL;
 	*siglen = 0;
@@ -230,8 +231,14 @@ static int sign_blob(const struct sshkey *pubkey, u_char ** sig, size_t *siglen,
 	else if (flags & SSH_AGENT_RSA_SHA2_512)
 		algo = "rsa-sha2-512";
 
-	if (sshkey_private_deserialize(tmpbuf, &prikey) != 0 ||
-	    sshkey_sign(prikey, sig, siglen, blob, blen, algo, NULL, NULL, 0) != 0) {
+	if (sshkey_private_deserialize(tmpbuf, &prikey) != 0) {
+		debug("cannot deserialize key");
+		goto done;
+	}
+	if (sshkey_is_sk(prikey))
+		sk_provider = "internal";
+	if (sshkey_sign(prikey, sig, siglen, blob, blen, algo, sk_provider,
+	    NULL, 0) != 0) {
 		debug("cannot sign using retrieved key");
 		goto done;
 	}
@@ -472,4 +479,4 @@ int process_keyagent_request(struct sshbuf* request, struct sshbuf* response, st
 	}
 }
 
-#pragma warning(pop)
+#pragma warning(pop)