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/movepick.h

@@ -41,6 +41,7 @@ struct Stats {
 
   static const Value Max = Value(1 << 28);
 
+  Stats() { clear(); }
   const T* operator[](Piece pc) const { return table[pc]; }
   T* operator[](Piece pc) { return table[pc]; }
   void clear() { std::memset(table, 0, sizeof(table)); }