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/thresholdselector.cc

@@ -79,7 +79,7 @@ ThresholdSelector::ThresholdSelector(double minValue, double maxValue, double de
     assert(minValue < maxValue);
 #endif
 
-    bgCurveProvider = NULL;
+    bgCurveProvider = nullptr;
     separatedSliders = false;
     initalEq1 = startAtOne;
     minValTop = minValBottom = minValue;
@@ -123,7 +123,7 @@ ThresholdSelector::ThresholdSelector(double minValue, double maxValue, double de
     assert(minValue < maxValue);
 #endif
 
-    bgCurveProvider = NULL;
+    bgCurveProvider = nullptr;
     separatedSliders = false;
     initalEq1 = startAtOne;
     minValTop = minValBottom = minValue;