Captcha not properly saving a correct answer in $_SESSION (fixed version)

Issue Summary

I think that there is an error in captcha.module validation function.
Even when a math answer is correct it comes with an error message.
Captcha seem to return an empty value.
I modified captcha.module as below and it works fine for me.

/**
* Default implementation of the captcha validation function.
*/

function captcha_captchavalidate(&$captcha_word, &$correct) {
$captcha_word = drupal_strtolower($captcha_word);M

if (($_SESSION['captcha'] != '') AND ($captcha_word == $_SESSION['captcha'])) {
$correct = TRUE;
}elseif (($_SESSION['captcha'] != '') AND ($captcha_word != $_SESSION['captcha'])) {
$correct = FALSE;
form_set_error('captcha_response', t('The answer you entered to the math problem is incorrect.'));

}
}