fix issue 1322, accept -1 as default payload-type for redundant coding (FEC).
fix issue 1322, accept -1 as default payload-type for redundant coding (FEC).

issue=1322 
test=trybot, voe auto-tes
Review URL: https://webrtc-codereview.appspot.com/1043007

git-svn-id: 917f5d3ca488f358c4d40eaec14422cf392ccec9@3446 4adac7df-926f-26a2-2b94-8c16560cd09d

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