scrolled window: Avoid a crash

scrolled window: Avoid a crash

gtk_scrolled_window_allocate_scrollbar was calling
gtk_scrolled_window_allocate_child just to get the relative
allocation, overlooking the fact that that function is only
safe to call if the scrolled window _has_ a child. Unfortunately,
gtk_scrolled_window_allocate_scrollbar will sometimes get called
when that is not the case. Since we are really only interested
in the relative allocation, just get that directly. This
fixes a segfault in the style-properties-nth-child reftest.