rtpstats: Don't save the ts & seqnum if the avg is not updated
rtpstats: Don't save the ts & seqnum if the avg is not updated

This makes it update correctly when you have more than one packet per
frame.

GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good