修复sdp中没ssrc时触发的bug

webrtc/Sdp.cpp

@@ -841,8 +841,10 @@ void RtcSession::loadFrom(const string &str, bool check) {
             if (rtc_ssrc_map.size() > 1) {
                 throw std::invalid_argument("sdp中不存在a=ssrc-group:FID字段,但是ssrc却有多个");
             }
+            if (rtc_ssrc_map.size() == 1) {
                 ssrc_rtp = rtc_ssrc_map.begin()->second.ssrc;
             }
+        }
         for (auto &pr : rtc_ssrc_map) {
             auto &rtc_ssrc = pr.second;
             if (rtc_ssrc.ssrc == ssrc_rtp) {
@@ -982,6 +984,7 @@ string RtcSession::toRtspSdp() const{
         }
     }
 
+    copy.session_name = "zlmediakit rtsp stream from webrtc";
     auto sdp = copy.toRtcSessionSdp();
     toRtsp(sdp->items);
     int i = 0;