This change lets the fuzzer modify the first few bytes of the RTP payload. One of the benefits is that it can cover the RED header splitter functionality. The CL also fixes an issue found while running the fuzzer locally. Bug: webrtc:11640 Change-Id: I7ca73676440897a14a0aaca796f70d381e016575 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/185819 Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org> Reviewed-by: Sam Zackrisson <saza@webrtc.org> Cr-Commit-Position: refs/heads/master@{#32242}