Fix crash on multiple feedback messages are received on old transport.

webrtc/modules/congestion_controller/delay_based_bwe.cc

Index: webrtc/modules/congestion_controller/delay_based_bwe.cc
diff --git a/webrtc/modules/congestion_controller/delay_based_bwe.cc b/webrtc/modules/congestion_controller/delay_based_bwe.cc
index 1a22f357c737e7a6bda3af17b15f6862a0507526..8f8618190c103946f534ca5cc5a43701ff316729 100644
--- a/webrtc/modules/congestion_controller/delay_based_bwe.cc
+++ b/webrtc/modules/congestion_controller/delay_based_bwe.cc
@@ -277,6 +277,7 @@ DelayBasedBwe::DelayBasedBwe(RtcEventLog* event_log, const Clock* clock)
 DelayBasedBwe::Result DelayBasedBwe::IncomingPacketFeedbackVector(
     const std::vector<PacketFeedback>& packet_feedback_vector) {
   RTC_DCHECK(network_thread_.CalledOnValidThread());
+  RTC_DCHECK(!packet_feedback_vector.empty());
 
   std::vector<PacketFeedback> sorted_packet_feedback_vector;
   SortPacketFeedbackVector(packet_feedback_vector,