Apply modernize-use-nullptr

Setup:
- `mkdir tidy; cd tidy`
- `cmake .. -DCMAKE_BUILD_TYPE=debug -DPROC_TARGET_NUMBER=1 -DCACHE_NAME_SUFFIX=4 -DBINDIR=. -DDATADIR=. -DBUILD_BUNDLE=ON -DWITH_LTO=OFF -DOPTION_OMP=OFF -DCMAKE_EXPORT_COMPILE_COMMANDS=ON`
- `cd ..`
- `find -name '*.cc' -exec clang-tidy-3.8 -header-filter=.* -p=tidy -fix-errors -checks=modernize-use-nullptr {} \;`

rtgui/toolpanel.cc

@@ -65,7 +65,7 @@ void ToolParamBlock::on_style_changed (const Glib::RefPtr<Gtk::Style>& style)
     updateStyle();
 }
 
-FoldableToolPanel::FoldableToolPanel(Gtk::Box* content, Glib::ustring toolName, Glib::ustring UILabel, bool need11, bool useEnabled) : ToolPanel(toolName, need11), parentContainer(NULL), exp(NULL), lastEnabled(true)
+FoldableToolPanel::FoldableToolPanel(Gtk::Box* content, Glib::ustring toolName, Glib::ustring UILabel, bool need11, bool useEnabled) : ToolPanel(toolName, need11), parentContainer(nullptr), exp(nullptr), lastEnabled(true)
 {
     if (!content) {
         return;