You'd need to edit poll.php. So download it from your server if you don't already have it on your local computer, then open it with a programming editor (like notepad++ - if you use wordpad or something like that it can cause problems). Next I would save a copy somewhere, then edit it and go to line 688. You should see code like is posted above. Make it look something like the second code (the indenting isn't important). Then upload it to your server, overwriting the one that's there. Then check if polls are working on your site, and if they aren't just upload the unedited copy you saved.
ETA: ...BTW, looks like it's a known bug that'll be fixed in the next version
http://tracker.vbulletin.com/browse/VBIV-13614 .