QHeaderView - length returns wrong value fix

QHeaderView - length returns wrong value fix

setSectionHidden called with (logindex, true) will
sometimes prevent a (correct) call to
resizeSection(logicalIndex, 0).
This seems a bit odd - however it does execute
d->doDelayedResizeSections().
Therefore the section is going to be hidden later.

However it is a problem that the length meanwhile is wrong.
(That is a value that is not the sum of the sections)

This is fixed by execute updates before returning the
length.

Task-number: QTBUG-14242

Change-Id: Ia1d2f6db3213792b250a6a37942b56554261cd3a
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>