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 {} \;`

rtengine/ipvibrance.cc

@@ -77,7 +77,7 @@ void ImProcFunctions::vibrance (LabImage* lab)
 
 //  int skip=1; //scale==1 ? 1 : 16;
     bool skinCurveIsSet = false;
-    DiagonalCurve* dcurve = NULL;
+    DiagonalCurve* dcurve = nullptr;
     dcurve = new DiagonalCurve (params->vibrance.skintonescurve, CURVES_MIN_POLY_POINTS);
 
     if (dcurve) {
@@ -85,14 +85,14 @@ void ImProcFunctions::vibrance (LabImage* lab)
             skinCurveIsSet = true;
         } else {
             delete dcurve;
-            dcurve = NULL;
+            dcurve = nullptr;
         }
     }
 
     if (!skinCurveIsSet && !params->vibrance.pastels && !params->vibrance.saturated) {
         if (dcurve) {
             delete dcurve;
-            dcurve = NULL;
+            dcurve = nullptr;
         }
 
         return;
@@ -117,7 +117,7 @@ void ImProcFunctions::vibrance (LabImage* lab)
 
     if (dcurve) {
         delete dcurve;
-        dcurve = NULL;
+        dcurve = nullptr;
     }
 
 
@@ -180,7 +180,7 @@ void ImProcFunctions::vibrance (LabImage* lab)
 
 
 #ifdef _DEBUG
-    MunsellDebugInfo* MunsDebugInfo = NULL;
+    MunsellDebugInfo* MunsDebugInfo = nullptr;
 
     if (avoidcolorshift) {
         MunsDebugInfo = new MunsellDebugInfo();