external ref assignment

This commit disables compile time "uninitialized variable error" for pyregr testsuite. And fixes reassignment bug, this code causes sigsegv:

```python
def crashme():
    class special():
        def __del__(self):
            print c
    c = (special(),)
    c = 123
```

Interesting thing: this code used to crash original python interpreter some time ago now it's our turn)