Don't free rtpinfo GString when it is NULL
Don't free rtpinfo GString when it is NULL

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=723554

surround-io/gst-rtsp-server,Lachann/gst-rtsp-server,thaytan/gst-rtsp-server,barisdemiray/gst-rtsp-server,hsteinhaus/gst-rtsp-server,surround-io/gst-rtsp-server,hsteinhaus/gst-rtsp-server,Lachann/gst-rtsp-server,hsteinhaus/gst-rtsp-server,surround-io/gst-rtsp-server,thaytan/gst-rtsp-server,Lachann/gst-rtsp-server,barisdemiray/gst-rtsp-server,barisdemiray/gst-rtsp-server,thaytan/gst-rtsp-server