Allow Position::init() to be called multiple times.

For the rationale to allow this, see commit
a66c73deef

This was broken when cuckoo hashing was added, and
subtly broke (for example) lichess' Android application,
thus illustrating the original judgement was sound.

No functional change.

src/position.cpp

@@ -168,6 +168,8 @@ void Position::init() {
   Zobrist::noPawns = rng.rand<Key>();
 
   // Prepare the cuckoo tables
+  std::memset(cuckoo, 0, sizeof(cuckoo));
+  std::memset(cuckooMove, 0, sizeof(cuckooMove));
   int count = 0;
   for (Piece pc : Pieces)
       for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)