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 {} \;`

rtgui/saveformatpanel.cc

@@ -20,7 +20,7 @@
 #include "multilangmgr.h"
 #include "guiutils.h"
 
-SaveFormatPanel::SaveFormatPanel () : listener (NULL)
+SaveFormatPanel::SaveFormatPanel () : listener (nullptr)
 {
 
     jpegqual = new Adjuster (M("SAVEDLG_JPEGQUAL"), 0, 100, 1, 100);
@@ -96,7 +96,7 @@ void SaveFormatPanel::init (SaveFormat &sf)
 {
 
     FormatChangeListener* tmp = listener;
-    listener = NULL;
+    listener = nullptr;
 
     if (sf.format == "jpg") {
         format->set_active (0);