Set simulcastIdx field to zero even if it has no meaning.
Helps to be able to memcmp between 2 parses of the same packet.

R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/30279004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7773 4adac7df-926f-26a2-2b94-8c16560cd09d