Panic on transport close

[tarrencev]

2020-07-16 21:17:09.122 INF WebRTCTransport.Close t.ID()=ckcpan12w005c0122fiq0ctfc
2020-07-16 21:17:09.122 INF Router.Close
2020-07-16 21:17:09.122 INF delRouter id=ckcpan12w005c0122fiq0ctfc
2020-07-16 21:17:09.122 ERR JitterBuffer.pliLoop j.Pub.WriteRTCP err=io: read/write on closed pipe
2020-07-16 21:17:09.464 ERR JitterBuffer.rembLoop j.Pub.WriteRTCP err=io: read/write on closed pipe
2020-07-16 21:17:09.757 ERR JitterBuffer.pliLoop j.Pub.WriteRTCP err=io: read/write on closed pipe
2020-07-16 21:17:10.058 INF webrtc ice closed for mid: ckcpan12w005c0122fiq0ctfc
2020-07-16 21:17:10.058 INF WebRTCTransport.Close t.ID()=ckcpankdc005j0122gxd47kfh
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x847350]

goroutine 9155 [running]:
github.com/pion/ion-sfu/pkg/rtc/transport.(*WebRTCTransport).Close(0xc000a50a20)
	/go/src/github.com/pion/ion-sfu/pkg/rtc/transport/webrtctransport.go:146 +0xc0
github.com/pion/ion-sfu/pkg/node.Subscribe.func1(0x6)
	/go/src/github.com/pion/ion-sfu/pkg/node/subscribe.go:53 +0xb6
created by github.com/pion/webrtc/v2.(*PeerConnection).onICEConnectionStateChange
	/go/pkg/mod/github.com/pion/webrtc/v2@v2.2.20/peerconnection.go:301 +0x12a