snowflake_server.httpHandler.ln is not initialized, leading to panic in oneshotMode

There are a few of these in the snowflake-server log:

2022/02/01 16:44:11 http: panic serving [scrubbed]: runtime error: invalid memory address or nil pointer dereference
goroutine 2513699 [running]:
net/http.(*conn).serve.func1(0xc007576dc0)
        /usr/lib/go-1.15/src/net/http/server.go:1801 +0x147
panic(0x7ea7a0, 0xb0c450)
        /usr/lib/go-1.15/src/runtime/panic.go:975 +0x47a
git.torproject.org/pluggable-transports/snowflake.git/v2/server/lib.(*SnowflakeListener).queueConn(0x0, 0x8dc820, 0xc04b943740, 0xc03557fbb0, 0xc04b9436e0)
        ./snowflake/server/lib/snowflake.go:275 +0x37
git.torproject.org/pluggable-transports/snowflake.git/v2/server/lib.oneshotMode(...)
        ./snowflake/server/lib/http.go:117
git.torproject.org/pluggable-transports/snowflake.git/v2/server/lib.(*httpHandler).ServeHTTP(0xc0001a0e80, 0x8d9800, 0xc057a75880, 0xc0dda2cd00)
        ./snowflake/server/lib/http.go:105 +0x5b7
net/http.serverHandler.ServeHTTP(0xc0001ee0e0, 0x8d9800, 0xc057a75880, 0xc0dda2cd00)
        /usr/lib/go-1.15/src/net/http/server.go:2843 +0xa3
net/http.(*conn).serve(0xc007576dc0, 0x8da1c0, 0xc013b86b00)
        /usr/lib/go-1.15/src/net/http/server.go:1925 +0x8ad
created by net/http.(*Server).Serve
        /usr/lib/go-1.15/src/net/http/server.go:2969 +0x36c

oneshotMode is called with httpHandler.ln as the *SnowflakeListener argument:

https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/blob/e828b0607662c7325f4d1bbf4c5072ce81f38fb9/server/lib/http.go#L100-105

		// We didn't find a matching token, which means that we are
		// dealing with a client that doesn't know about such things.
		// "Unread" the token by constructing a new Reader and pass it
		// to the old one-session-per-WebSocket mode.
		conn2 := &overrideReadConn{Conn: conn, Reader: io.MultiReader(bytes.NewReader(token[:]), conn)}
		err = oneshotMode(conn2, addr, handler.ln)

httpHandler is:

https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/blob/e828b0607662c7325f4d1bbf4c5072ce81f38fb9/server/lib/http.go#L63-68

type httpHandler struct {
	// pconn is the adapter layer between stream-oriented WebSocket
	// connections and the packet-oriented KCP layer.
	pconn *turbotunnel.QueuePacketConn
	ln    *SnowflakeListener
}

But in the only place httpHandler is instantiated, its ln field is left uninitialized:

https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/blob/e828b0607662c7325f4d1bbf4c5072ce81f38fb9/server/lib/snowflake.go#L80-85

	handler := httpHandler{
		// pconn is shared among all connections to this server. It
		// overlays packet-based client sessions on top of ephemeral
		// WebSocket connections.
		pconn: turbotunnel.NewQueuePacketConn(addr, clientMapTimeout),
	}