sample_neighbors() core dumps when sampling from unknown ids

## 🐛 Bug

`sample_neighbors()` core dumps instead of throwing exceptions when sampling from unknown ids. If unknown id is small, id=3, then sometimes exception are thrown. If unknown id=102012, it seem to always core.

## To Reproduce

```
import dgl
g = dgl.graph(([0, 0, 1, 1, 2, 2], [1, 2, 0, 1, 2, 0]))
sg = dgl.sampling.sample_neighbors(g, [3,102012], 1)

```

## Expected behavior

Throw exception instead of core dump

## Environment

- `dgl.__version__` = 0.8.2
- cpu
- OS: Linux