bugfix: assert walrus (":=") combo side-eff. breaks w/ asserts disabled

```
$ python3 -O
Python 3.10.6 (main, Mar 10 2023, 10:55:28) [GCC 11.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> assert (x := 2)
>>> x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined
```

pity. it looked to be a neat and concise pattern.

electrum/plugins/jade/jade.py

@@ -280,7 +280,8 @@ class Jade_KeyStore(Hardware_KeyStore):
             change = [None] * len(tx.outputs())
             for index, txout in enumerate(tx.outputs()):
                 if txout.is_mine and txout.is_change:
-                    assert (desc := txout.script_descriptor)
+                    desc = txout.script_descriptor
+                    assert desc
                     if is_multisig:
                         # Multisig - wallet details must be registered on Jade hw
                         multisig_name = _register_multisig_wallet(wallet, self, txout.address)