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/init.cc

@@ -35,7 +35,7 @@ namespace rtengine
 
 const Settings* settings;
 
-MyMutex* lcmsMutex = NULL;
+MyMutex* lcmsMutex = nullptr;
 
 int init (const Settings* s, Glib::ustring baseDir, Glib::ustring userSettingsDir)
 {