193 kill process

Fixes https://github.com/backdrop/backdrop-issues/issues/193
This array_key_exists() isn't entirely safe (assuming it's needed in the first place) because it only checks one level of depth but there are two keys in the array. This would be safer:

```php
$output .= (isset($variables['item_attributes'][$delta]) ? backdrop_attributes($variables['item_attributes'][$delta]) : '');
```