Fixes crash where packets were sent to a receive stream that had been destroyed but not removed from the ssrc mapping from call to receiver. Added a repro case that reliably crashed before the fix. BUG= R=stefan@webrtc.org Review URL: https://webrtc-codereview.appspot.com/2161007 git-svn-id: http://webrtc.googlecode.com/svn/trunk@4681 4adac7df-926f-26a2-2b94-8c16560cd09d