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/loadinitial.cc

@@ -36,7 +36,7 @@ InitialImage* InitialImage::load (const Glib::ustring& fname, bool isRaw, int* e
 
     isrc->setProgressListener (pl);
 
-    if(isRaw && pl == NULL) {
+    if(isRaw && pl == nullptr) {
         *errorCode = isrc->load (fname, true);
     } else {
         *errorCode = isrc->load (fname);
@@ -44,7 +44,7 @@ InitialImage* InitialImage::load (const Glib::ustring& fname, bool isRaw, int* e
 
     if (*errorCode) {
         delete isrc;
-        return NULL;
+        return nullptr;
     }
 
     return isrc;