This is formally defined as a signed 24-bit value in RFC 3550 section 6.4.1.
See RFC 3550 Appendix A.3 for the reason why it may turn negative.
Noticed on discuss-webrtc mailing list.
BUG=webrtc:8626
Change-Id: I7317f73e9490a876e8445bd3d6b66095ce53ca0a
Reviewed-on: https://webrtc-review.googlesource.com/30901
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21142}