QFileSystemModel: add an assert to prevent leaks

Private::addNode() adds the new node to the parent's hash without
checking whether a node already exists. If it does, then the old
node is leaked.

Add an assertion to check for this condition.

Change-Id: Ib949d71c86c28d1152a538756c0efc283c793c24
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/widgets/dialogs/qfilesystemmodel.cpp

@@ -1698,6 +1698,7 @@ QFileSystemModelPrivate::QFileSystemNode* QFileSystemModelPrivate::addNode(QFile
             node->volumeName = QString::fromWCharArray(name);
     }
 #endif
+    Q_ASSERT(!parentNode->children.contains(fileName));
     parentNode->children.insert(fileName, node);
     return node;
 }