Fix for issue #2832

The patch fixes the issue breaking highlighting when a smarty code is written within an HTML attribute. However, there's still an issue with Smarty variables within a scope of a comment. Just look at `mode/smartymixed/index.html`:
```
  {* Multiline smarty
  * comment, no {$variables} here
  *}
```
The last line is not highlighted here. It should be a similar kind of issue.