jquery.footHovzer (debugging toolbar): Don't cause a scrollbar to appear if content is shorter than window height (T38273)

When the debugging toolbar is enabled ($wgDebugToolbar = true;) and the page does not fill the full screen, useless scrollbar appears, at least with the Vector skin (it allows scrolling down about the height of the debug toolbar). Fix this! :)

The issue affects the jquery.footHovzer module.

Please read the complete bug description and comments on the bug report (T38273), as they are often very helpful. Students are required to read Wikimedia's general instructions first.