Fix rare crash on HT collisions

Due to a race, or even a collision (single threaded), the he.eval can be wrong.
And so we cannot deduce that we are not in check by trusting this value. And, of
course, doing a null move in check is a disaster, the opponent will capture our
king, and the engine crashes soon after.

Hopefully Demolito is now ready for 176 threads in TCEC without crashes :)

src/position.c

@@ -314,7 +314,9 @@ void pos_move(Position *pos, const Position *before, move_t m)
 
     // Capture piece on to square (if any)
     if (capture != NB_PIECE) {
+        assert(capture != KING);
         pos->rule50 = 0;
+
         // Use pos_color_on() instead of them, because we could be playing a KxR castling here
         clear_square(pos, pos_color_on(pos, to), capture, to);
 

src/search.c

@@ -281,9 +281,11 @@ static int search(Worker *worker, const Position *pos, int ply, int depth, int a
     }
 
     // Null search
-    if (depth >= 2 && !pvNode
+    if (depth >= 2 && !pvNode && !pos->checkers
             && staticEval >= beta && pos->pieceMaterial[us].eg) {
-        assert(!pos->checkers);
+        // Note that staticEval >= beta should exclude the case where we are in check (eval() cannot be called in
+        // check and a value -MATE is assigned). But with HT races or even HT collisions in single threaded case,
+        // one cannot make any assumptions on HT data.
         const int nextDepth = depth - (3 + depth / 4) - (refinedEval >= beta + 167);
 
         pos_switch(&nextPos, pos);