message: fix unstable test case UdpListenerImplTest.SendData
  If Network::Test::readFromSocket() return error code 'Again', `data.addresses_.peer_`
  is null, `data.addresses_.peer_->asString()` causes crash because it dereferences
  a null pointer.
diff: @@ -411,8 +411,6 @@ TEST_P(UdpListenerImplTest, SendData) {
  client_socket_->ioHandle(), *client_socket_->localAddress(), data);
  bytes_read = result.rc_;
  - EXPECT_EQ(send_from_addr->asString(), data.addresses_.peer_->asString());
  -
  if (bytes_read >= bytes_to_read || retry == 10 ||
  result.err_->getErrorCode() != Api::IoError::IoErrorCode::Again) {
  break;
  @@ -423,6 +421,7 @@ TEST_P(UdpListenerImplTest, SendData) {
  ASSERT(bytes_read == 0);
  } while (true);
  EXPECT_EQ(bytes_to_read, bytes_read);
  + EXPECT_EQ(send_from_addr->asString(), data.addresses_.peer_->asString());
  EXPECT_EQ(data.buffer_->toString(), payload);
  }
  