Fixed incorrect parameter passed to LoginForm::userBlockedMessage()

The error is Catchable fatal error: Argument 1 passed to
LoginForm::userBlockedMessage() must be an instance of Block, null given.

The problem is that User::$mBlock is not always the correct block, when
checking that a blocked logged-in user can create an account.

bug: 47138
Change-Id: I43f35cfa342737f05a08a693a86949c14348321f
Merge "Fixed incorrect parameter passed to LoginForm::userBlockedMessage()"