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/previewhandler.cc

@@ -23,7 +23,7 @@
 using namespace rtengine;
 using namespace rtengine::procparams;
 
-PreviewHandler::PreviewHandler () : image(NULL), previewScale(1.)
+PreviewHandler::PreviewHandler () : image(nullptr), previewScale(1.)
 {
 
     pih = new PreviewHandlerIdleHelper;
@@ -121,7 +121,7 @@ int delImageUI (void* data)
     if (pih->phandler->image) {
         IImage8* oldImg = pih->phandler->image;
         oldImg->getMutex().lock ();
-        pih->phandler->image = NULL;
+        pih->phandler->image = nullptr;
         oldImg->getMutex().unlock ();
     }