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/curveeditor.cc

@@ -196,10 +196,10 @@ CurveEditor::CurveEditor (Glib::ustring text, CurveEditorGroup* ceGroup, CurveEd
     bgHistValid = false;
     remoteDrag = false;
     selected = DCT_Linear;
-    bottomBarCP = NULL;
-    leftBarCP = NULL;
-    curveCP = NULL;
-    relatedWidget = NULL;
+    bottomBarCP = nullptr;
+    leftBarCP = nullptr;
+    curveCP = nullptr;
+    relatedWidget = nullptr;
 
     group = ceGroup;
     subGroup = ceSubGroup;