Fix error of undefined file. Force return when no more files to check.

This PR fully fixes the error:
```
Fatal error: Unable to read 'undefined' file (Error code: ENOENT).
```

You call `done()`, but it doesn't fully end the `validate` function. The `return` ensures that it exits the function to end the grunt task.

I came across this error when linting 2 partial html files using the `wrapfile` option. My grunt config looked something like this:
``` js
validation: {
            options: {
                wrapfile: 'test/validation-wrap.html',
                reportpath: false,
                reset: true
            },
            files: {
                src: ['partials/**/*.html']
            }
        },
```