Fixed a bug where the MaximumSizeHint of a layout with spans was wrong

This was spotted while tracking down a similar bug related to spans.

The problem was only for the maximum size, since the size of an ignored
row/column was min: 0, pref: 0, max: FLT_MAX (the default constructed
values for a QGridLayoutBox).

Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
(cherry picked from qtbase/9d7ae6dfbe25fb70a362a4cf955c187cd24cb007)

Change-Id: Ic0f3cf12639fbb6ab85c4946d9e8cc1d8d68e753
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>