Fix a crash bug in QuicUnackedPacketMap when truncated acks arrived.

net/quic/quic_unacked_packet_map.cc

Index: net/quic/quic_unacked_packet_map.cc
diff --git a/net/quic/quic_unacked_packet_map.cc b/net/quic/quic_unacked_packet_map.cc
index 4d2bec982d25af0dda22b3009735d926d1e58f14..7276b139e91bc7f223c40504c98187819a2c93a4 100644
--- a/net/quic/quic_unacked_packet_map.cc
+++ b/net/quic/quic_unacked_packet_map.cc
@@ -117,9 +117,24 @@ void QuicUnackedPacketMap::ClearPreviousRetransmissions(size_t num_to_clear) {
       break;
     }
 
-    info->all_transmissions->pop_front();
-    LOG_IF(DFATAL, info->all_transmissions->empty())
-        << "Previous retransmissions must have a newer transmission.";
+    if (info->all_transmissions != NULL) {
+      if (info->all_transmissions->size() < 2) {
+        LOG(DFATAL) << "all_transmissions must be NULL or have multiple "
+                    << "elements.  size:" << info->all_transmissions->size();
+        delete info->all_transmissions;
+      } else {
+        info->all_transmissions->pop_front();
+        if (info->all_transmissions->size() == 1) {
+          // Set the newer transmission's 'all_transmissions' entry to NULL.
+          QuicPacketSequenceNumber new_transmission =
+              info->all_transmissions->front();
+          TransmissionInfo* new_info =
+              &unacked_packets_.at(new_transmission - least_unacked_);
+          delete new_info->all_transmissions;
+          new_info->all_transmissions = NULL;
+        }
+      }
+    }
     unacked_packets_.pop_front();
     ++least_unacked_;
     --num_to_clear;