Panic in nested exceptions handling

```python
try:
    {}[1]
except KeyError:
    try:
        raise RuntimeError()
    except RuntimeError:
        pass
```

### expected result
silent and clean process finish

### current result
```
thread 'main' panicked at 'assertion failed: vm.pop_exception().is_some()',
.../.cargo/git/checkouts/rustpython-f8ef4d934ac33cd8/73edde6/vm/src/frame.rs:883:21
```