Fix a bug in generate_piece_checks()

We are generating also king moves that give check !

Of course these moves are illegal so are in any case
filtered out in MovePicker. Neverthless we should avoid
to generate them.

Also simplify a bit the code.

No functional change.

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

src/movegen.cpp

@@ -848,10 +848,8 @@ namespace {
 
     // Direct checks
     b = target & ~dc;
-    if (Piece != KING || b)
-    {
     Bitboard checkSqs = pos.attacks_from<Piece>(ksq) & pos.empty_squares();
-        if (!checkSqs)
+    if (Piece == KING || !checkSqs)
         return mlist;
 
     while (b)
@@ -865,7 +863,6 @@ namespace {
         Bitboard bb = pos.attacks_from<Piece>(from) & checkSqs;
         SERIALIZE_MOVES(bb);
     }
-    }
     return mlist;
   }