Unclosed braces cause hanging

These examples will run forever

```cpp
void f() {
    int x = 0;
```
```cpp
{
    int x = 0;
```

Setups tried:
- `xeus-cpp==0.8.0` on conda-forge for MacOS arm64 (M3)
- `xeus-cpp==0.8.0` on exmscripten-forge for emscripten-wasm32