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/shcselector.cc

@@ -21,7 +21,7 @@
 #include "multilangmgr.h"
 #include "mycurve.h"
 
-SHCSelector::SHCSelector() : ColoredBar(RTO_Left2Right), movingPosition(-1), tmpX(0.0), tmpPos(0.0), wslider(0.0), cl(NULL)
+SHCSelector::SHCSelector() : ColoredBar(RTO_Left2Right), movingPosition(-1), tmpX(0.0), tmpPos(0.0), wslider(0.0), cl(nullptr)
 {
 
     positions[0] = defaults[0] = 0.25;