authserver round robin and multiple NAS-IP-Address avpairs

Hello, when configuring round-robin authservers in sendserver.c function "rc_send_server" the NAS-IP-Address gets added for each fail-overed server resulting in the NAS-IP-Address avpair being sent multiple times. I fixed it by checking for presence of NAS-IP-Address avpair (see patch). Rgds, Stefan