TreeModel: Add a few more soft asserts
TreeModel: Add a few more soft asserts

This establishes an additional invariant, namely that items
that are not part of a model have m_model == 0, even if their
destruction is imminent.

Change-Id: I0bad7fff57d21ddcefe06953cd0da8c2897baa4a
Reviewed-by: Christian Stenger <accbb51712d7b9c4fb108439d01e716148e5f9e6@theqtcompany.com>
Reviewed-by: Eike Ziller <c142da16c73a95a82d35191bcc55b22835617128@theqtcompany.com>

xianian/qt-creator,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,xianian/qt-creator,xianian/qt-creator,xianian/qt-creator,martyone/sailfish-qtcreator,xianian/qt-creator,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,xianian/qt-creator,martyone/sailfish-qtcreator,xianian/qt-creator,xianian/qt-creator,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,xianian/qt-creator