message: Fix WriteFlowControl test
  Summary: The assignment is wrong
diff: @@ -597,7 +597,7 @@ TEST_F(QuicTransportTest, WriteFlowControl) {
  loopForWrites();
  EXPECT_EQ(conn.outstandings.packets.size(), 1);
  - auto& packet =
  + const auto& packet =
  getFirstOutstandingPacket(conn, PacketNumberSpace::AppData)->packet;
  bool blockedFound = false;
  bool dataBlockedFound = false;
  @@ -654,9 +654,10 @@ TEST_F(QuicTransportTest, WriteFlowControl) {
  // Verify that there is one Data Blocked frame emitted.
  EXPECT_EQ(conn.outstandings.packets.size(), num_outstandings + 2);
  - packet = getLastOutstandingPacket(conn, PacketNumberSpace::AppData)->packet;
  + const auto& packet2 =
  + getLastOutstandingPacket(conn, PacketNumberSpace::AppData)->packet;
  dataBlockedFound = false;
  - for (auto& frame : packet.frames) {
  + for (auto& frame : packet2.frames) {
  auto dataBlocked = frame.asDataBlockedFrame();
  if (!dataBlocked) {
  continue;
  