Fix GetStats() crash.

GetStats() can be called before codecs are set and the underlying
webrtc::VideoSendStream is created, leading to a null-pointer
dereference.

BUG=1788
R=pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/14099004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/talk@6876 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed