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/whitebalance.cc

@@ -147,7 +147,7 @@ static double wbTemp2Slider(double temp)
     return sval;
 }
 
-WhiteBalance::WhiteBalance () : FoldableToolPanel(this, "whitebalance", M("TP_WBALANCE_LABEL")), wbp(NULL), wblistener(NULL)
+WhiteBalance::WhiteBalance () : FoldableToolPanel(this, "whitebalance", M("TP_WBALANCE_LABEL")), wbp(nullptr), wblistener(nullptr)
 {
 
     Gtk::HBox* hbox = Gtk::manage (new Gtk::HBox ());
@@ -833,7 +833,7 @@ WBEntry* WhiteBalance::findWBEntry (Glib::ustring label, enum WB_LabelType lblTy
         }
     }
 
-    return NULL;
+    return nullptr;
 }
 
 int WhiteBalance::_setActiveMethod(Glib::ustring &label, Gtk::TreeModel::Children &children)