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/filepanel.cc

@@ -28,7 +28,7 @@ int FilePanelInitUI (void* data)
     return 0;
 }
 
-FilePanel::FilePanel () : parent(NULL)
+FilePanel::FilePanel () : parent(nullptr)
 {
 
     dirpaned = Gtk::manage ( new Gtk::HPaned () );
@@ -227,7 +227,7 @@ bool FilePanel::fileSelected (Thumbnail* thm)
     pendingLoadMutex.lock();
     pendingLoad *pl = new pendingLoad();
     pl->complete = false;
-    pl->pc = 0;
+    pl->pc = nullptr;
     pl->thm = thm;
     pendingLoads.push_back(pl);
     pendingLoadMutex.unlock();