QDirModel: replace a copy by a reference

Since we are storing pointers to the elements of the
vector thus copied, we better make sure we don't cause
it to deep-copy. The old code was fragile in that sense,
using a reference removes one layer of potential mess-ups.

Change-Id: Ib8ebbb47c2a478b5e666e767d05429700b528afd
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/widgets/itemviews/qdirmodel.cpp

@@ -168,7 +168,7 @@ void QDirModelPrivate::invalidate()
     while (!nodes.empty()) {
         const QDirNode *current = nodes.pop();
         current->stat = false;
-        const QVector<QDirNode> children = current->children;
+        const QVector<QDirNode> &children = current->children;
         for (int i = 0; i < children.count(); ++i)
             nodes.push(&children.at(i));
     }