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/cplx_wavelet_level.h

@@ -189,7 +189,7 @@ void wavelet_level<T>::destroy(T ** subbands)
             delete[] subbands[1];
         } else {
             for(int j = 1; j < 4; j++) {
-                if(subbands[j] != NULL) {
+                if(subbands[j] != nullptr) {
                     delete[] subbands[j];
                 }
             }