Use signed arithmetic for indexes that can be negative Bug: webrtc:15523 Change-Id: I41ea47a679242c8febe70373529173733058c479 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/322120 Reviewed-by: Jakob Ivarsson <jakobi@webrtc.org> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org> Reviewed-by: Per Åhgren <peah@webrtc.org> Commit-Queue: Sergey Sukhanov <sergeysu@webrtc.org> Cr-Commit-Position: refs/heads/main@{#40924}