Error out when trying to masquerading as blocked user

New patch applying to current 7.x-1.x-dev and 7.x-1.0-rc7

Includes:

• Require users.status = 1 in masquerade_autocomplete() and masquerade_autocomplete_multiple(), so you do not get any blocked users via autocomplete.
• Validate user's status in masquerade_block_1_validate(), so you will get a form validation error if you entered a blocked user.
• Check user's status in masquerade_switch_user().

I do not think that we need to log to watchdog (above patches did).

I have one question: the other patches checked user's status like:

if (!empty($new_user->uid) && empty($new_user->status)) { ... }

What's the purpose of !empty($new_user->uid) &&?
My patch just checks empty($new_user->status).