Sequential user login attempts result in a notice

[Alexander Obuhovich]

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.
4. 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

Task: http://tracker.in-portal.org/view.php?id=1378

Ready for testing.

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