Free multicastMemberships on UDP endpoint close

tcpip.Endpoint.Close is documented to free all resources associated
with an endpoint so we don't need to create an empty map to clear
the multicast memberships.

PiperOrigin-RevId: 390609826

pkg/tcpip/transport/udp/endpoint.go

@@ -266,7 +266,7 @@ func (e *endpoint) Close() {
 	for mem := range e.multicastMemberships {
 		e.stack.LeaveGroup(e.NetProto, mem.nicID, mem.multicastAddr)
 	}
-	e.multicastMemberships = make(map[multicastMembership]struct{})
+	e.multicastMemberships = nil
 
 	// Close the receive list and drain it.
 	e.rcvMu.Lock()