Infinite loop when serializing feature flags

### How do you use Sentry?

Sentry Saas (sentry.io)

### Version

2.18

### Steps to Reproduce

```python
from sentry_sdk._lru_cache import LRUCache

cache = LRUCache(3)
cache.set(0, 0)

copied = copy(cache)
copied.set(2, 2)
cache.get_all()  # spins forever
```

### Expected Result

The loop breaks and flags are returned.

### Actual Result

Infinite loop.