move_{do,undo}: FIX forgotten king[] update, add few bug_on()

src/move-gen.c

@@ -109,7 +109,7 @@ bool pseudo_is_legal(const pos_t *pos, const move_t move)
  * @start is set to next non-checked move in pseudo-legal list.
  * Position pseudo-legal moves must be already calculated before calling this function.
  *
- * @return: move, or -1 if no move.
+ * @return: move, or MOVE_NONE if no move.
  */
 move_t pos_next_legal(const pos_t *pos, int *start)
 {