Don't attempt to restore old TOCModel if it hasn't been actually saved

Because if it hasn't been saved (54b4a20e07)
this results in m_model == 0, which makes us crash.

ui/toc.cpp

@@ -100,6 +100,9 @@ void TOC::prepareForReload()
 
 void TOC::rollbackReload()
 {
+    if( !m_model->hasOldModelData() )
+        return;
+
     TOCModel *m = m_model;
     m_model = m->clearOldModelData();
     delete m;

ui/tocmodel.cpp

@@ -370,6 +370,11 @@ void TOCModel::setOldModelData( TOCModel *model, const QVector<QModelIndex> &lis
     d->m_oldTocExpandedIndexes = list;
 }
 
+bool TOCModel::hasOldModelData() const
+{
+    return (d->m_oldModel != 0);
+}
+
 TOCModel *TOCModel::clearOldModelData() const
 {
     TOCModel *oldModel = d->m_oldModel;

ui/tocmodel.h

@@ -45,6 +45,7 @@ class TOCModel : public QAbstractItemModel
         bool isEmpty() const;
         bool equals( const TOCModel *model ) const;
         void setOldModelData( TOCModel *model, const QVector<QModelIndex> &list );
+        bool hasOldModelData() const;
         TOCModel *clearOldModelData() const;
 
         QString externalFileNameForIndex( const QModelIndex &index ) const;