Prevent DCHECK failure in QUIC congestion control

I saw a DCHECK failure in GeneralLossAlgorithm::DetectLosses
where least_in_flight_ was being compared even though it was uninitialized.
I'm unable to reproduce this reliably but this CL looks correct from code inspection.

gfe-relnote: n/a, no behavior change
PiperOrigin-RevId: 304603107
Change-Id: I8557403886143d763fd6c77be3a7ecbf76628ae2

quic/core/congestion_control/general_loss_algorithm.cc

diff --git a/quic/core/congestion_control/general_loss_algorithm.cc b/quic/core/congestion_control/general_loss_algorithm.cc
index 6c09013..7354f50 100644
--- a/quic/core/congestion_control/general_loss_algorithm.cc
+++ b/quic/core/congestion_control/general_loss_algorithm.cc
@@ -31,7 +31,7 @@
     const AckedPacketVector& packets_acked,
     LostPacketVector* packets_lost) {
   loss_detection_timeout_ = QuicTime::Zero();
-  if (!packets_acked.empty() &&
+  if (!packets_acked.empty() && least_in_flight_.IsInitialized() &&
       packets_acked.front().packet_number == least_in_flight_) {
     if (packets_acked.back().packet_number == largest_newly_acked &&
         least_in_flight_ + packets_acked.size() - 1 == largest_newly_acked) {