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/popupcommon.cc

@@ -26,8 +26,8 @@
 
 PopUpCommon::PopUpCommon (Gtk::Button* thisButton, const Glib::ustring& label)
     : selected (-1) // -1 means that the button is invalid
-    , menu (0)
-    , buttonImage (0)
+    , menu (nullptr)
+    , buttonImage (nullptr)
 {
     button = thisButton;
     hasMenu = false;