Fix assertion failed

```
17926 Thread 'IO Worker #1' panicked at 'assertion failed: `(left != right)`
17927   left: `None`,
17928  right: `None`', sync/src/block/extension.rs:193
```

The full log I got while testing "sync › empty block › nodeA creates 3 empty blocks": [57xdkszmgy.log](https://github.com/CodeChain-io/codechain/files/2373759/57xdkszmgy.log)

It occurs with a low probability.