New user dialog can grow and push Forward button out of the netbook screen

[Roman Yepishev]

On my Acer Aspire One ubuntu-sso looks as the screenshot netbook-create-account.png shows.
When I press Forward w/o filling any items the window grows beyond the display height (600px) and Forward button cannot be reached as seen on netbook-create-account2.png
In case at least 2 labels telling that "field is required" pop up, forward button becomes hidden from the user and it is impossible to make that window shrink again.