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/thumbbrowserbase.cc

@@ -25,7 +25,7 @@
 using namespace std;
 
 ThumbBrowserBase::ThumbBrowserBase ()
-    : lastClicked(NULL), previewHeight(options.thumbSize), numOfCols(1), inspector(NULL), isInspectorActive(false), location(THLOC_FILEBROWSER)
+    : lastClicked(nullptr), previewHeight(options.thumbSize), numOfCols(1), inspector(nullptr), isInspectorActive(false), location(THLOC_FILEBROWSER)
 {
     inW = -1;
     inH = -1;
@@ -728,7 +728,7 @@ void ThumbBrowserBase::on_style_changed (const Glib::RefPtr<Gtk::Style>& style)
     refreshThumbImages ();
 }
 
-ThumbBrowserBase::Internal::Internal () : ofsX(0), ofsY(0), parent(NULL), dirty(true)
+ThumbBrowserBase::Internal::Internal () : ofsX(0), ofsY(0), parent(nullptr), dirty(true)
 {
 }
 
@@ -774,7 +774,7 @@ void ThumbBrowserBase::buttonPressed (int x, int y, int button, GdkEventType typ
 {
     // GUI already acquired
 
-    ThumbBrowserEntryBase* fileDescr = NULL;
+    ThumbBrowserEntryBase* fileDescr = nullptr;
     bool handled = false;
 
     {