fix-half-clock-bug (#187)

Elo   | -0.43 +- 2.32 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=32MB
LLR   | 2.95 (-2.94, 2.94) [-5.00, 0.00]
Games | N: 42412 W: 10447 L: 10499 D: 21466
Penta | [243, 4805, 11164, 4749, 245]
http://chess.grantnet.us/test/34986/
Elo   | -0.05 +- 2.75 (95%)
SPRT  | 40.0+0.40s Threads=1 Hash=64MB
LLR   | 2.97 (-2.94, 2.94) [-5.00, 0.00]
Games | N: 29840 W: 7259 L: 7263 D: 15318
Penta | [59, 3381, 8032, 3401, 47]
http://chess.grantnet.us/test/34993/
bench: 4103067

src/chess/board.cc

@@ -766,7 +766,7 @@ board board::forward_(const move& mv) const noexcept {
   copy.lat_.them<c>().clear_ep_mask();
   ++copy.lat_.ply_count;
   ++copy.lat_.half_clock;
-  if (mv.is_capture() || mv.piece() == piece_type::pawn) { copy.lat_.half_clock = 0; }
+  if (!mv.is_null() && (mv.is_capture() || mv.piece() == piece_type::pawn)) { copy.lat_.half_clock = 0; }
   return copy;
 }