Ensure "pass" parameter is present during mod-rewrite url parsing

Steps to reproduce:

1. Enable mod-rewrite.
2. Try pressing "Login" button (on a login form) twice in a row without entering any user credentials.
3. You should end with "/?env=u--2----" at the end of url.
3. Try to login with valid credentials.
5. Warnings below are shown.

Notice (#1): Undefined index: pass in ...\core\units\helpers\user_helper.php on line 426
Warning (#2): implode() [function.implode]: Invalid arguments passed in ...\core\units\helpers\user_helper.php on line 426