mollom fails to validate user/register captcha after an initial form_set_error

I have a user/register form that contains some extra required fields, e.g. full name, terms and conditions checkbox, and so on. I've encountered an issue with a form error and mollom validation. The steps to reproduce are:

• User goes to user/register and doesn't enter in a value for all the required fields
• User enters captcha correctly and submits
• the expected error about the missing field appears
• user enters the missing value, and correctly fills in new captcha
• user submits the form, but gets the error "The entered CAPTCHA solution is not correct. We generated a new CAPTCHA so please try again."
• This error keeps repeating no matter how many times the user enters the captcha correctly

I think it's something to do with the session id associated with the 1st captcha is different from the subsequent ones. The attached patch fixes the issue, though there may be a better solution.

Cheers,
Stella