Prevent OOB reads for zero-length H264 payloads.

Also fixes zero-length OOB reads for generic packetization.

BUG=webrtc:4771
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1218013002

Cr-Commit-Position: refs/heads/master@{#9521}
3 files changed