message: Remove unchecked back() call that could be called on an empty collection
  Summary: Calling back() on an empty collection is undefined. This avoids doing it for this case which is just a log line.
diff: @@ -90,10 +90,7 @@ calculateAlarmDuration(const QuicConnectionStateBase& conn) {
  adjustedAlarmDuration = folly::chrono::ceil<std::chrono::milliseconds>(
  lastSentPacketTime + alarmDuration - now);
  } else {
  - auto lastSentPacketNum =
  - conn.outstandings.packets.back().packet.header.getPacketSequenceNum();
  VLOG(10) << __func__ << " alarm already due method=" << *alarmMethod
  - << " lastSentPacketNum=" << lastSentPacketNum
  << " lastSentPacketTime="
  << lastSentPacketTime.time_since_epoch().count()
  << " now=" << now.time_since_epoch().count()
  