Don't crash when project has no layers

This only crashes when Qt is built in
debug mode because there is an assert
in the item model class that checks for
index range validity.

src/core/qgsmaplayermodel.cpp

@@ -154,17 +154,20 @@ void QgsMapLayerModel::removeLayers( const QStringList &layerIds )
 
 void QgsMapLayerModel::addLayers( const QList<QgsMapLayer *> &layers )
 {
-  int offset = 0;
-  if ( mAllowEmpty )
-    offset++;
-
-  beginInsertRows( QModelIndex(), mLayers.count() + offset, mLayers.count() + layers.count() - 1  + offset );
-  Q_FOREACH ( QgsMapLayer *layer, layers )
+  if ( layers.count( ) )
   {
-    mLayers.append( layer );
-    mLayersChecked.insert( layer->id(), Qt::Unchecked );
+    int offset = 0;
+    if ( mAllowEmpty )
+      offset++;
+
+    beginInsertRows( QModelIndex(), mLayers.count() + offset, mLayers.count() + layers.count() - 1  + offset );
+    Q_FOREACH ( QgsMapLayer *layer, layers )
+    {
+      mLayers.append( layer );
+      mLayersChecked.insert( layer->id(), Qt::Unchecked );
+    }
+    endInsertRows();
   }
-  endInsertRows();
 }
 
 QModelIndex QgsMapLayerModel::index( int row, int column, const QModelIndex &parent ) const