derivekey: don't require --neuter for xpub input

Previously, if xpub was passed, the tool crashed without --neuter option.
Now it is optional if the input is an xpub.

Author: starius

cmd/chantools/derivekey.go

@@ -107,7 +107,7 @@ func deriveKey(extendedKey *hdkeychain.ExtendedKey, path string,
 	}
 
 	privKey, xPriv := na, na
-	if !neuter {
+	if !neuter && wif != nil {
 		privKey, xPriv = wif.String(), child.String()
 	}
 

cmd/chantools/derivekey_test.go

@@ -123,3 +123,23 @@ func TestDeriveKeyXpub(t *testing.T) {
 	h.assertLogContains("03dc8655d58bd4fd4326863fe34bd5cdddbefaa3b042571" +
 		"05eb1ab99aa05e01c2a")
 }
+
+func TestDeriveKeyXpubNoNeuter(t *testing.T) {
+	h := newHarness(t)
+
+	// Derive a specific key from xpub.
+	derive := &deriveKeyCommand{
+		Path: "m/5/6",
+		rootKey: &rootKey{
+			RootKey: "xpub661MyMwAqRbcFtXgS5sYJABqqG9YLmC4Q1Rdap" +
+				"9gSE8NqtwybGhePY2gZ29ESFjqJoCu1Rupje8YtGqse" +
+				"fD265TMg7usUDFdp6W1EGMcet8",
+		},
+	}
+
+	err := derive.Execute(nil, nil)
+	require.NoError(t, err)
+
+	h.assertLogContains("03dc8655d58bd4fd4326863fe34bd5cdddbefaa3b042571" +
+		"05eb1ab99aa05e01c2a")
+}