Fix score_captures() for the case of capture promotions

In case we have more than one promotion move, prefer
the one that captures the biggest piece.

Almost no functional change, anyhow I don't expect any
ELO change, it is just the correct thing to do.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/movepick.cpp

@@ -224,11 +224,11 @@ void MovePicker::score_captures() {
   for (MoveStack* cur = moves; cur != lastMove; cur++)
   {
       m = cur->move;
-      if (move_is_promotion(m))
-          cur->score = QueenValueMidgame;
-      else
       cur->score =  pos.midgame_value_of_piece_on(move_to(m))
                   - pos.type_of_piece_on(move_from(m));
+
+      if (move_is_promotion(m))
+          cur->score += QueenValueMidgame;
   }
 }