Prevent __proto__ var in linted script from causing an error.

Saw this error when linting [getter-in-prototype.js](https://github.com/joyent/node/blob/master/deps/v8/test/mjsunit/getter-in-prototype.js):

```
TypeError: Object #<Object> has no method 'push'
    at note_implied (/home/andreas/.local/lib/node_modules/jshint/src/jshint.js:1960:6)
    at Object.state.syntax.(identifier).nud (/home/andreas/.local/lib/node_modules/jshint/src/jshint.js:2046:5)
    at expression (/home/andreas/.local/lib/node_modules/jshint/src/jshint.js:961:30)
    at statement (/home/andreas/.local/lib/node_modules/jshint/src/jshint.js:1684:7)
    at statements (/home/andreas/.local/lib/node_modules/jshint/src/jshint.js:1733:12)
    at itself (/home/andreas/.local/lib/node_modules/jshint/src/jshint.js:4717:5)
    at lint (/home/andreas/.local/lib/node_modules/jshint/src/cli.js:290:7)
    at exports.run.verbose (/home/andreas/.local/lib/node_modules/jshint/src/cli.js:401:4)
    at Array.forEach (native)
    at Object.exports.run (/home/andreas/.local/lib/node_modules/jshint/src/cli.js:390:9)
```

Added a quick fix that prevents it from erroring, but probably isn't the correct fix for actually making a `__proto__` var subject to linting.