xua_rkm: send_reg_req: Avoid adding Routing Context with value 0
We use routing context 0 internally as "no routing context". Since the Routing Context in the Routing KEy IE in RKM REG REQ is optional, if local rctx is 0 (because we expect to get one allocated by the STP and receive it through RKM REG RESP), avoid sending it with the value 0.
