Enable easy move detection only for recaptures

It could lead to terrible mistakes otherwise, as
it happened during a game on playchess when on
this position (after white's f4):

2q4r/4b1k1/p3rpp1/3np2p/PpNpNP1P/1P1P2PQ/2P1R3/4R1K1 b - - 0 1

SF moves immediately e5xf4 instead of the correct f5.
In general during engine matches it is impossible the
opponent leaves a piece hanging or anyhow starts a
clear losing sequence. So avoid to fall in subtle traps.

No functional change.

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

src/position.cpp

@@ -1265,7 +1265,7 @@ int Position::see(Move m) const {
   {
       Square capQq = to - pawn_push(sideToMove);
 
-      assert(capturedType == NO_PIECE_TYPE);
+      assert(!capturedType);
       assert(type_of(piece_on(capQq)) == PAWN);
 
       // Remove the captured pawn