Prevent heap overflows for incorrect FEC packet lengths.

Bugs found by manual inspection of code, not by fuzzing or packet
replays. At least one of them confirmed by local fuzzing.

BUG=chromium:496094, webrtc:4771
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1182793002

Cr-Commit-Position: refs/heads/master@{#9542}
3 files changed