rtpmp2tpay: do not try to flush a packet when no data is available
rtpmp2tpay: do not try to flush a packet when no data is available

https://bugzilla.gnome.org/show_bug.cgi?id=668874

freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,matsu/gst-plugins-good,matsu/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,matsu/gst-plugins-good,matsu/gst-plugins-good