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/cache.h

@@ -64,7 +64,7 @@ public:
         virtual void onDestroy() = 0;
     };
 
-    Cache(unsigned long _size, Hook* _hook = 0) :
+    Cache(unsigned long _size, Hook* _hook = nullptr) :
         store_size(_size),
         hook(_hook)
     {