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/distortion.cc

@@ -26,7 +26,7 @@ using namespace rtengine::procparams;
 Distortion::Distortion (): FoldableToolPanel(this, "distortion", M("TP_DISTORTION_LABEL"))
 {
 
-    rlistener = NULL;
+    rlistener = nullptr;
     autoDistor = Gtk::manage (new Gtk::Button (M("TP_DISTORTION_AUTO")));
     autoDistor->set_image (*Gtk::manage (new RTImage ("distortion-auto.png")));
     autoDistor->set_tooltip_text (M("TP_DISTORTION_AUTO_TIP"));