Fix possibly uninitialized value

I'm not sure about this one. It's possible that there's something
happening behind the scenes that I don't understand. But let's just
initialize this to 0 and be sure.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

qt-ui/models.cpp

@@ -906,6 +906,7 @@ QVariant TreeItem::data(int column, int role) const
 
 TreeModel::TreeModel(QObject *parent) : QAbstractItemModel(parent)
 {
+	columns = 0; // I'm not sure about this one - I can't see where it gets initialized
 	rootItem = new TreeItem();
 }