rtpsession: Sometimes tries to send empty RTCP packets

Submitted by Sebastian Dröge @slomo

Link to original bug (#759119)

Description

See http://lists.freedesktop.org/archives/gstreamer-commits/2015-November/090796.html

This should be debugged before the next release and needs a way to reproduce it still. Worst case we should make this an assertion before the release if nobody has time to debug it.

The session should never ever try to send an empty packet, if it does either the application does things wrong or there is a bug in rtpsession.