Fixed a bug that the training data generation crashes if eval_limit is high.

src/learn/learner.cpp

@@ -634,11 +634,19 @@ void MultiThinkGenSfen::thread_worker(size_t thread_id)
 					// reach leaf
 					// cout << pos;
 
-					auto v = Eval::evaluate(pos);
+					Value v;
+					if (pos.checkers()) {
+						// HACK: If a king is checkmated, Eval::evalute(pos) crashes with an
+						// assertion.  To avoid crashes, return value1 instead.
+						v = value1;
+					}
+					else {
+						v = Eval::evaluate(pos);
 						// evaluate() returns the evaluation value on the turn side, so
 						// If it's a turn different from root_color, you must invert v and return it.
 						if (rootColor != pos.side_to_move())
 							v = -v;
+					}
 
 					// Rewind.
 					// Is it C++x14, and isn't there even foreach to turn in reverse?