Do not replace connection ID for unknown versions

This prevents the QUIC_BUG from b/139661722 from firing.

gfe-relnote: avoid a GFE_BUG when parsing a specific invalid packet

Startblock:
  after 2019-09-24 08:00 in US/Pacific
PiperOrigin-RevId: 270909914
Change-Id: Idbeb2ee5c02c6d011d0696ad1cfb4aa64547fcb9

quic/core/quic_dispatcher.cc

diff --git a/quic/core/quic_dispatcher.cc b/quic/core/quic_dispatcher.cc
index 4d7a2d7..b130fe9 100644
--- a/quic/core/quic_dispatcher.cc
+++ b/quic/core/quic_dispatcher.cc
@@ -401,8 +401,14 @@
     it->second->ProcessUdpPacket(packet_info.self_address,
                                  packet_info.peer_address, packet_info.packet);
     return true;
-  } else {
+  } else if (packet_info.version.transport_version !=
+             QUIC_VERSION_UNSUPPORTED) {
     // We did not find the connection ID, check if we've replaced it.
+    // This is only performed for supported versions because packets with
+    // unsupported versions can flow through this function in order to send
+    // a version negotiation packet, but we know that their connection ID
+    // did not get replaced since that is performed on connection creation,
+    // and that only happens for known verions.
     QuicConnectionId replaced_connection_id = MaybeReplaceServerConnectionId(
         server_connection_id, packet_info.version);
     if (replaced_connection_id != server_connection_id) {