panic: invalid conn id length

[marten-seemann]

This was introduced by #3529.

MITM test with QUIC version v2 unsuccessful attacks corrupting packets 
  downloads a message when packet are corrupted towards the server
  /home/circleci/project/integrationtests/self/mitm_test.go:246
panic: invalid conn id length

goroutine 46470 [running]:
github.com/lucas-clemente/quic-go/internal/protocol.ParseConnectionID(...)
	/home/circleci/project/internal/protocol/connection_id.go:52
github.com/lucas-clemente/quic-go/internal/wire.ParseConnectionID({0xc004158600?, 0x22c6df02e8?, 0x0?}, 0x56579f?)
	/home/circleci/project/internal/wire/header.go:35 +0x22f
github.com/lucas-clemente/quic-go.(*packetHandlerMap).handlePacket(0xc007b67d90, 0xc004409f80)
	/home/circleci/project/packet_handler_map.go:380 +0x65
github.com/lucas-clemente/quic-go.(*packetHandlerMap).listen(0xc007b67d90)
	/home/circleci/project/packet_handler_map.go:375 +0xc5
created by github.com/lucas-clemente/quic-go.newPacketHandlerMap
	/home/circleci/project/packet_handler_map.go:140 +0x385