Fix crash when trying to place toolbar into zero-height window.

Change QToolBarAreaLayout::item() to return a pointer
and check return values in plug().

Task-number: QTBUG-37183
Change-Id: I7029eb9739cbe603460e87d3e5493f116bdb3a89
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
(cherry picked from qtbase/e38ad9455099a83e2a8619f19ca949bc64ae6f82)