Make low ply history size fixed

Size of low ply history should always be the same, so ensure it.

closes https://github.com/official-stockfish/Stockfish/pull/5630

No functional change

src/movepick.cpp

@@ -179,7 +179,7 @@ void MovePicker::score() {
                         : pt == ROOK && bool(to & threatenedByMinor) ? 24335
                                                                      : 0);
 
-            if (ply < 4)
+            if (ply < LOW_PLY_HISTORY_SIZE)
                 m.value += 8 * (*lowPlyHistory)[ply][m.from_to()] / (1 + 2 * ply);
         }