Micro optimize generate_piece_checks()

Avoid calculating piece attacks if there aren't
available check sqaures for the given piece.

About 15% of cases. Not a biggie but still something
especially in the middle game where king is well covered
inside his castle.

No functional change.

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

src/movegen.cpp

@@ -815,6 +815,9 @@ namespace {
         return mlist;
 
     Bitboard checkSqs = pos.piece_attacks<Piece>(ksq) & pos.empty_squares();
+    if (!checkSqs)
+        return mlist;
+
     while (b)
     {
         Square from = pop_1st_bit(&b);