b/f: set rtp_interface correctly on RTP stream creation.

core/AmSession.h

@@ -620,6 +620,8 @@ inline AmRtpAudio* AmSession::RTPStream() {
   if (NULL == _rtp_str.get()) {
     DBG("creating RTP stream instance for session [%p]\n", 
 	this);
+    if(rtp_interface < 0)
+      rtp_interface = dlg.getOutboundIf();
     _rtp_str.reset(new AmRtpAudio(this,rtp_interface));
   }
   return _rtp_str.get();