member field + lambda results unexpected UnboundLocalError

Code:
```
class X:
    v = 10
    f = lambda x=v: x
```

Expected:
silence

Actual:
```
Traceback (most recent call last):
  File "test_lambda_scope.py", line 3, in <module>
    f = lambda x=v: x
  File "test_lambda_scope.py", line 3, in X
    f = lambda x=v: x
UnboundLocalError: local variable 'v' referenced before assignment
```