Restore check for nil writePipe in WebRTCPeer.Close.

I removed this check in 047d3214 because
NewWebRTCPeer always initializes writePipe, and it is never reset to
nil. However tests used &WebRTCPeer{} which bypasses NewWebRTCPeer and
leaves writePipe set to nil.

https://bugs.torproject.org/34049#comment:3
https://bugs.torproject.org/34050

client/lib/webrtc.go

@@ -236,7 +236,9 @@ func exchangeSDP(broker *BrokerChannel, offer *webrtc.SessionDescription) *webrt
 // Close all channels and transports
 func (c *WebRTCPeer) cleanup() {
 	// Close this side of the SOCKS pipe.
-	c.writePipe.Close()
+	if c.writePipe != nil { // c.writePipe can be nil in tests.
+		c.writePipe.Close()
+	}
 	if nil != c.transport {
 		log.Printf("WebRTC: closing DataChannel")
 		c.transport.Close()