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/previewloader.cc

@@ -42,7 +42,7 @@ public:
 
         Job():
             dir_id_(0),
-            listener_(0)
+            listener_(nullptr)
         {}
 
         int dir_id_;
@@ -122,7 +122,7 @@ public:
         // if something got
 // Issue 2406       FileBrowserEntry* fdn = 0;
         try {
-            Thumbnail* tmb = 0;
+            Thumbnail* tmb = nullptr;
             {
                 if (Glib::file_test(j.dir_entry_, Glib::FILE_TEST_EXISTS)) {
                     tmb = cacheMgr->getEntry(j.dir_entry_);
@@ -176,7 +176,7 @@ PreviewLoader* PreviewLoader::getInstance(void)
 void PreviewLoader::add(int dir_id, const Glib::ustring& dir_entry, PreviewLoaderListener* l)
 {
     // somebody listening?
-    if ( l != 0 ) {
+    if ( l != nullptr ) {
         {
             MyMutex::MyLock lock(impl_->mutex_);