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/previewwindow.cc

@@ -21,7 +21,7 @@
 #include "imagearea.h"
 #include "cursormanager.h"
 
-PreviewWindow::PreviewWindow () : previewHandler(NULL), mainCropWin(NULL), imageArea(NULL), imgX(0), imgY(0), imgW(0), imgH(0), zoom(0.0), isMoving(false), needsUpdate(false)
+PreviewWindow::PreviewWindow () : previewHandler(nullptr), mainCropWin(nullptr), imageArea(nullptr), imgX(0), imgY(0), imgW(0), imgH(0), zoom(0.0), isMoving(false), needsUpdate(false)
 {
 
     rconn = signal_size_allocate().connect( sigc::mem_fun(*this, &PreviewWindow::on_resized) );