Fix read of property "use strict" of undefined.

Fixes jshint crash with:

```js
// jshint esnext: true, globalstrict: true
"use strict";
var a = () => 42;
```
Slightly cleaner solution.
Merge pull request #1052 from usrbincc/use-strict-of-undefined

Fix "TypeError: Cannot read property 'use strict' of undefined".