In includes/message_parser.php, the poll title length is checked two times, once for 100 characters, and again (after a call to the parse function) for 255 characters. This was included in revision 7036 but seems unnecessary:
The parse function is called with the parameter update_this_message set to false, so to quote a comment in the code: "$this->message won't be altered, the text will be returned instead".
The database is capable of storing at least 255 characters, so maybe only check for that.

I noticed this while editing a poll which was posted some time ago in phpBB2 - the poll title is longer than 100 characters and already stored in the database, but phpBB3 does not allow to save the edited poll (i only want to set it from sticky to normal. leaving the title and everything else).