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_dec.cc

@@ -26,7 +26,7 @@ namespace rtengine
 wavelet_decomposition::~wavelet_decomposition()
 {
     for(int i = 0; i <= lvltot; i++) {
-        if(wavelet_decomp[i] != NULL) {
+        if(wavelet_decomp[i] != nullptr) {
             delete wavelet_decomp[i];
         }
     }