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/image8.h

@@ -51,7 +51,7 @@ public:
         return 8 * sizeof(unsigned char);
     }
     virtual void         getScanline (int row, unsigned char* buffer, int bps);
-    virtual void         setScanline (int row, unsigned char* buffer, int bps, float *minValue = NULL, float *maxValue = NULL);
+    virtual void         setScanline (int row, unsigned char* buffer, int bps, float *minValue = nullptr, float *maxValue = nullptr);
 
     // functions inherited from IImage*:
     virtual MyMutex&     getMutex ()