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/thumbimageupdater.cc

@@ -47,10 +47,10 @@ public:
         {}
 
         Job():
-            tbe_(0),
-            priority_(NULL),
+            tbe_(nullptr),
+            priority_(nullptr),
             upgrade_(false),
-            listener_(0)
+            listener_(nullptr)
         {}
 
         ThumbBrowserEntryBase* tbe_;
@@ -156,7 +156,7 @@ public:
 
         // unlock and do processing; will relock on block exit, then call listener
         double scale = 1.0;
-        rtengine::IImage8* img = 0;
+        rtengine::IImage8* img = nullptr;
         Thumbnail* thm = j.tbe_->thumbnail;
 
         if ( j.upgrade_ ) {
@@ -205,7 +205,7 @@ void
 ThumbImageUpdater::add(ThumbBrowserEntryBase* tbe, bool* priority, bool upgrade, ThumbImageUpdateListener* l)
 {
     // nobody listening?
-    if ( l == 0 ) {
+    if ( l == nullptr ) {
         return;
     }