sphinx: Actually use the full pubkey in ECDH key generation

The spec says that we use the libsecp256k1 style ECDH, which uses the
full compressed pubkey from the scalar multiplication which is then
hashed. This is in contrast to the btcsuite implementation which was
only using the hashed X-coordinate.

daemon/sphinx.c

@@ -272,7 +272,7 @@ static bool create_shared_secret(
 	secp256k1_ec_pubkey_serialize(secp256k1_ctx, ecres, &outputlen,
 				      &pkcopy, SECP256K1_EC_COMPRESSED);
 	struct sha256 h;
-	sha256(&h, ecres + 1, sizeof(ecres) - 1);
+	sha256(&h, ecres, sizeof(ecres));
 	memcpy(secret, &h, sizeof(h));
 	return true;
 }
@@ -361,7 +361,7 @@ static struct hop_params *generate_hop_params(
 			secp256k1_ctx, der, &outputlen, &temp,
 			SECP256K1_EC_COMPRESSED);
 		struct sha256 h;
-		sha256(&h, der + 1, sizeof(der) - 1);
+		sha256(&h, der, sizeof(der));
 		memcpy(&params[i].secret, &h, sizeof(h));
 
 		compute_blinding_factor(