QmlDesigner.dragAndDrop: crash fix

This fixes a crash if we drag in complex items with children.

Change-Id: I0e924d3130cbd43dc9a20469576dce8a0f67f3df
Reviewed-on: http://codereview.qt.nokia.com/1715


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>