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/thumbbrowserentrybase.cc

@@ -24,10 +24,10 @@
 ThumbBrowserEntryBase::ThumbBrowserEntryBase (const Glib::ustring& fname)
     : fnlabw(0), fnlabh(0), dtlabw(0), dtlabh(0), exlabw(0), exlabh(0), prew(0), preh(0),
       prex(0), prey(0), upperMargin(6), borderWidth(1), textGap(6), sideMargin(8), lowerMargin(8),
-      preview(NULL), dispname(Glib::path_get_basename (fname)), buttonSet(NULL), width(0), height(0),
+      preview(nullptr), dispname(Glib::path_get_basename (fname)), buttonSet(nullptr), width(0), height(0),
       exp_width(0), exp_height(0), startx(0), starty(0), ofsX(0), ofsY(0), redrawRequests(0),
-      parent(NULL), original(NULL), bbSelected(false), bbFramed(false), bbPreview(NULL),
-      thumbnail(NULL), filename(fname), shortname(dispname), exifline(""), datetimeline(""),
+      parent(nullptr), original(nullptr), bbSelected(false), bbFramed(false), bbPreview(nullptr),
+      thumbnail(nullptr), filename(fname), shortname(dispname), exifline(""), datetimeline(""),
       selected(false), drawable(false), filtered(false), framed(false), processing(false), italicstyle(false),
       edited(false), recentlysaved(false), updatepriority(false), withFilename(WFNAME_NONE) {}
 
@@ -387,7 +387,7 @@ void ThumbBrowserEntryBase::resize (int h)
 
     if ( preh != old_preh || width != old_width ) {
         delete [] preview;
-        preview = NULL;
+        preview = nullptr;
         refreshThumbnailImage ();
     } else {
         backBuffer.clear();    // This will force a backBuffer update on queue_draw