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/crop.cc

@@ -38,7 +38,7 @@ public:
 Crop::Crop (): FoldableToolPanel(this, "crop", M("TP_CROP_LABEL"), false, true)
 {
 
-    clistener = NULL;
+    clistener = nullptr;
 
     maxw = 3000;
     maxh = 2000;