nesh crashes when tab completing at certain place

It's always reproducible on my machine. I tried to reproduce it with other code but failed. Currently the exact reproducible code is:

```javascript
function construct(f) {
let newObject = Object.create(f.prototype);
newO
```

The nesh will crash when you press tab key right after the "newO". Here's the screenshot:


Run nesh with the JavaScript snippet from the issue and press Tab immediately after "newO" to confirm the crash. Then trace the tab-completion entry point and reproduce the case in a regression test; done means completion no longer crashes at that position.

Written by the indexing model from the issue text.