modules_k/rtpproxy: fix crash related to SDP without ssession IP

 - when the IP address is provided only in the SDP stream description
and not in the SDP session description, the rtpproxy module was crashing
due to wrong check of the IP address length.
 - thanks to Daniel Constantin Mierla for reporting and investigating this.

modules_k/rtpproxy/rtpproxy.c

@@ -2074,7 +2074,7 @@ force_rtp_proxy(struct sip_msg* msg, char* str1, char* str2, int offer)
 			sdp_stream = get_sdp_stream(msg, sdp_session_num, sdp_stream_num);
 			if(!sdp_stream) break;
 
-			if (sdp_stream->ip_addr.s && !sdp_stream->ip_addr.len) {
+			if (sdp_stream->ip_addr.s && sdp_stream->ip_addr.len>0) {
 				oldip = sdp_stream->ip_addr;
 				pf = sdp_stream->pf;
 			} else {