Correct zero-init of Thread data members

If not explicitly initialized in a class constructor,
then all data members are default-initialized when
the corresponing struct/class is instanced.

For array and built-in types (int, char, etc..)
default-initialization is a no-op and we need to
explicitly zero them.

No functional change.

src/thread.cpp

@@ -37,8 +37,6 @@ Thread::Thread() {
   resetCalls = exit = false;
   maxPly = callsCnt = 0;
   tbHits = 0;
-  history.clear();
-  counterMoves.clear();
   idx = Threads.size(); // Start from 0
 
   std::unique_lock<Mutex> lk(mutex);