message: fix spurious assertion failure
diff: @@ -147,7 +147,7 @@ expr Byte::nonptr_value() const {
  expr Byte::is_poison(bool fullbit) const {
  expr np = nonptr_nonpoison();
  if (byte_has_ptr_bit() && bits_int_poison() == 1) {
  - assert(ptr_nonpoison().eq(np == 0));
  + assert(!np.isValid() || ptr_nonpoison().eq(np == 0));
  return np == 1;
  }
  return expr::mkIf(is_ptr(), !ptr_nonpoison(),
  