Fixed triggering of an assertion if the parent directory of the model has been...

Fixed triggering of an assertion if the parent directory of the model has been deleted from outside (reviewed by David Faure). This fixes a crash in Dolphin and Konqueror if the currently shown directory gets deleted from outside.

CCMAIL: faure@kde.org
CCMAIL: riccardo@kde.org

svn path=/trunk/KDE/kdelibs/; revision=729674