fix a potential nil-pointer panic.

[raulk]

Cherry-picked from #156, while we sort the discussion out.

[marten-seemann]

I don’t think this is necessary. The filtered conn is only created when we use a gater:

go-libp2p-quic-transport/reuse.go

Lines 27 to 30 in eda7485

	func newReuseConn(conn net.PacketConn, gater connmgr.ConnectionGater) *reuseConn {
	if gater != nil {
	conn = newFilteredConn(conn, gater)
	}

Can you please revert this PR?

[raulk]

Oh, right. The filteredConn is rather confusing.