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/toolpanelcoord.cc

@@ -516,7 +516,7 @@ void ToolPanelCoordinator::closeImage ()
 
     if (ipc) {
         ipc->stopProcessing ();
-        ipc = NULL;
+        ipc = nullptr;
     }
 }
 
@@ -646,7 +646,7 @@ void ToolPanelCoordinator::autoCropRequested ()
 rtengine::RawImage* ToolPanelCoordinator::getDF()
 {
     if (!ipc) {
-        return NULL;
+        return nullptr;
     }
 
     const rtengine::ImageMetaData *imd = ipc->getInitialImage()->getMetaData();
@@ -661,13 +661,13 @@ rtengine::RawImage* ToolPanelCoordinator::getDF()
         return rtengine::dfm.searchDarkFrame( maker, model, iso, shutter, timestamp);
     }
 
-    return NULL;
+    return nullptr;
 }
 
 rtengine::RawImage* ToolPanelCoordinator::getFF()
 {
     if (!ipc) {
-        return NULL;
+        return nullptr;
     }
 
     const rtengine::ImageMetaData *imd = ipc->getInitialImage()->getMetaData();
@@ -685,7 +685,7 @@ rtengine::RawImage* ToolPanelCoordinator::getFF()
         return rtengine::ffm.searchFlatField( maker, model, lens, focallength, aperture, timestamp);
     }
 
-    return NULL;
+    return nullptr;
 }
 
 Glib::ustring ToolPanelCoordinator::GetCurrentImageFilePath()
@@ -764,7 +764,7 @@ void ToolPanelCoordinator::foldAllButOne (Gtk::Box* parent, FoldableToolPanel* o
 {
 
     for (auto toolPanel : toolPanels) {
-        if (toolPanel->getParent() != NULL) {
+        if (toolPanel->getParent() != nullptr) {
             ToolPanel* currentTP = toolPanel;
 
             if (currentTP->getParent() == parent) {