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}