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/colorprovider.h

@@ -45,7 +45,7 @@ public:
     double ccGreen;
     double ccBlue;
 
-    ColorCaller() : colorCallerId(-1), colorProvider(NULL), ccRed(0.), ccGreen(0.), ccBlue(0.) {}
+    ColorCaller() : colorCallerId(-1), colorProvider(nullptr), ccRed(0.), ccGreen(0.), ccBlue(0.) {}
     void setColorProvider (ColorProvider* p, int id)
     {
         colorProvider = p;