message: into_result for cv_return_type should use receive_string_mut
diff: @@ -20,7 +20,7 @@ impl<S: Into<O>, O> cv_return_value<S, O> {
  if self.error_msg.is_null() {
  Ok(self.result.into())
  } else {
  - Err(Error::new(self.error_code, crate::templ::receive_string(self.error_msg)))
  + Err(Error::new(self.error_code, crate::templ::receive_string_mut(self.error_msg)))
  }
  }
  }
  