Sphere validation against CAPT yields segmentation fault

```py
import vamp

pc = [[0.1] * 3]

env = vamp.Environment()
env.add_pointcloud(pc, 0.0, 0.1, 0.01)

print(vamp.sphere.validate([0.0, 0.0, 0.0], env))  # should be False, segfaults instead
```