Fix bug in wallet generate in Qt when cancelled

Prior to this commit, if a user chose cancel while selecting a
filename for a new wallet in Qt, the wallet name was set to the
default `wallet.jmdat`, which could lead to a crash or overwrite.
This fixes that by ensuring that the wallet generation code
recognizes the cancel operation.

jmclient/jmclient/wallet_utils.py

@@ -523,6 +523,10 @@ def wallet_generate_recover_bip39(method, walletspath, default_wallet_name,
         return False
 
     wallet_name = callbacks[3]()
+    if wallet_name == "cancelled":
+        # currently used only by Qt, because user has option
+        # to click cancel in dialog.
+        return False
     if not wallet_name:
         wallet_name = default_wallet_name
     wallet_path = os.path.join(walletspath, wallet_name)

scripts/joinmarket-qt.py

@@ -1482,7 +1482,9 @@ class JMMainWindow(QMainWindow):
                                               QLineEdit.Normal, "wallet.jmdat")
         if not ok:
             JMQtMessageBox(self, "Create wallet aborted", mbtype='warn')
-            return None
+            # cannot use None for a 'fail' condition, as this is used
+            # for the case where the default wallet name is to be used in non-Qt.
+            return "cancelled"
         self.walletname = str(walletname)
         return self.walletname