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/coordinateadjuster.cc

@@ -125,7 +125,7 @@ void CoordinateAdjuster::createWidgets(const std::vector<Axis> &axis)
     set_spacing(3);
 
     for (unsigned int i = 0; i < count; ++i) {
-        AxisAdjuster *currAdjuster = NULL;
+        AxisAdjuster *currAdjuster = nullptr;
         const Axis *currAxis = &(axis.at(i));
         axisAdjusters.at(i) = new AxisAdjuster(this, currAxis, i);
         currAdjuster = axisAdjusters.at(i);