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/imagesource.h

@@ -63,7 +63,7 @@ protected:
 
 public:
     ImageSource () : references (1), redAWBMul(-1.), greenAWBMul(-1.), blueAWBMul(-1.),
-        embProfile(NULL), idata(NULL), dirpyrdenoiseExpComp(INFINITY) {}
+        embProfile(nullptr), idata(nullptr), dirpyrdenoiseExpComp(INFINITY) {}
 
     virtual ~ImageSource            () {}
     virtual int         load        (const Glib::ustring &fname, bool batch = false) = 0;
@@ -111,7 +111,7 @@ public:
     virtual bool        isRAW() const = 0;
     virtual DCPProfile* getDCP(const ColorManagementParams &cmp, ColorTemp &wb, DCPProfile::ApplyState &as)
     {
-        return NULL;
+        return nullptr;
     };
 
     virtual void        setProgressListener (ProgressListener* pl) {}