disable save button for partially signed tx

gui/qt/transaction_dialog.py

@@ -179,7 +179,8 @@ class TxDialog(QDialog, MessageBoxMixin):
 
     def sign(self):
         def sign_done(success):
-            if success:
+            # note: with segwit we could save partially signed tx, because they have a txid
+            if self.tx.is_complete():
                 self.prompt_if_unsaved = True
                 self.saved = False
                 self.save_button.setDisabled(False)