message: smt: fix crash in load(lambda) preprocessing
  as reported by Juneyoung in
diff: @@ -1288,8 +1288,10 @@ expr expr::load(const expr &idx) const {
  auto extract_load = [&idx](expr &e, const expr &var) {
  if (auto app = e.isAppOf(Z3_OP_SELECT)) { // load(array, idx)
  - assert(expr(Z3_get_app_arg(ctx(), app, 1)).eq(var));
  - e = expr(Z3_get_app_arg(ctx(), app, 0)).load(idx);
  + expr lambda_idx = Z3_get_app_arg(ctx(), app, 1);
  + expr new_idx = lambda_idx.subst(var, idx).simplify();
  + assert(!idx.isValid() || !lambda_idx.eq(new_idx));
  + e = expr(Z3_get_app_arg(ctx(), app, 0)).load(new_idx);
  return true;
  }
  return false;
  