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/labcurve.cc

@@ -623,8 +623,8 @@ void LCurve::setBatchMode (bool batchMode)
     chromaticity->showEditedCB ();
     rstprotection->showEditedCB ();
     curveEditorG->setBatchMode (batchMode);
-    lcshape->setBottomBarColorProvider(NULL, -1);
-    lcshape->setLeftBarColorProvider(NULL, -1);
+    lcshape->setBottomBarColorProvider(nullptr, -1);
+    lcshape->setLeftBarColorProvider(nullptr, -1);
 }