Prevent fuzzer from sending unsupported RETRY packets

This CL also adds some DCHECKs and QUIC_BUGs that helped debug this, and a trivial refactor in test code.

gfe-relnote: adds QUIC_BUG in send path, not flag protected
PiperOrigin-RevId: 272244952
Change-Id: Icb77bbb2e498ef02ddc13a839834ddf9d8ed7632

quic/test_tools/quic_test_utils.cc

diff --git a/quic/test_tools/quic_test_utils.cc b/quic/test_tools/quic_test_utils.cc
index dae5857..89cf15d 100644
--- a/quic/test_tools/quic_test_utils.cc
+++ b/quic/test_tools/quic_test_utils.cc
@@ -922,7 +922,8 @@
   if (!versions) {
     versions = &supported_versions;
   }
-  if (QuicVersionHasLongHeaderLengths((*versions)[0].transport_version) &&
+  ParsedQuicVersion version = (*versions)[0];
+  if (QuicVersionHasLongHeaderLengths(version.transport_version) &&
       version_flag) {
     header.retry_token_length_length = VARIABLE_LENGTH_INTEGER_LENGTH_1;
     header.length_length = VARIABLE_LENGTH_INTEGER_LENGTH_2;
@@ -932,7 +933,6 @@
   QuicFramer framer(*versions, QuicTime::Zero(), perspective,
                     kQuicDefaultConnectionIdLength);
   framer.SetInitialObfuscators(destination_connection_id);
-  ParsedQuicVersion version = (*versions)[0];
   EncryptionLevel level =
       header.version_flag ? ENCRYPTION_INITIAL : ENCRYPTION_FORWARD_SECURE;
   if (level != ENCRYPTION_INITIAL) {