Use nullptr instead of zero (0) as the null pointer constant

src/sync.cpp

@@ -162,7 +162,7 @@ void DeleteLock(void* cs)
         return;
     }
     boost::unique_lock<boost::mutex> lock(lockdata.dd_mutex);
-    std::pair<void*, void*> item = std::make_pair(cs, (void*)0);
+    std::pair<void*, void*> item = std::make_pair(cs, nullptr);
     LockOrders::iterator it = lockdata.lockorders.lower_bound(item);
     while (it != lockdata.lockorders.end() && it->first.first == cs) {
         std::pair<void*, void*> invitem = std::make_pair(it->first.second, it->first.first);