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/indclippedpanel.cc

@@ -76,7 +76,7 @@ void IndicateClippedPanel::buttonToggled ()
 
     // this will redraw the linked Before image area
     // which is set when before/after view is enabled
-    if (imageArea->iLinkedImageArea != NULL) {
+    if (imageArea->iLinkedImageArea != nullptr) {
         imageArea->iLinkedImageArea->queue_draw ();
     }
 }