rtpsession: source stats only gets last ssrc

Submitted by Aleix Conchillo Flaqué

Link to original bug (#728918)

Description

If an RTCP RR packet contains two SSRC (i.e. different fraction loss, etc.) The sender source will only get update with the last SSRC found in the packet.

The sender might need to adjust the sending bitrate according to this information which is not complete.