Fix sometimes incorrect key for prefetches

STC
https://tests.stockfishchess.org/tests/view/61737b4f6ce927be32558401
LLR: 2.95 (-2.94,2.94) <-2.50,0.50>
Total: 138712 W: 34914 L: 34942 D: 68856
Ptnml(0-2): 421, 14817, 38894, 14817, 407

Very minor tweak since Position::key() depends on the 50 move rule counter.
Comments: cddde31eed

closes https://github.com/official-stockfish/Stockfish/pull/3759

No functional change

src/position.cpp

@@ -1013,9 +1013,9 @@ void Position::do_null_move(StateInfo& newSt) {
   }
 
   st->key ^= Zobrist::side;
+  ++st->rule50;
   prefetch(TT.first_entry(key()));
 
-  ++st->rule50;
   st->pliesFromNull = 0;
 
   sideToMove = ~sideToMove;