AEC3: Fix render buffer headroom calculation on underruns

The read and write pointers can meet. Currently, this causes an overflow in the headroom calculation (headroom == buffer size) which offsets the buffers needlessly. This causes a loss of transparency when using the neural residual echo estimation module.

Tested:
  1. audioproc_f is bitexact on a large set of aecdumps, without ML-REE
  2. I see improved transparency on a dump with underruns, with ML-REE

Bug: b:487988676, webrtc:442444736
Change-Id: I902a20f28c603f85a1c10ae5b525788e3a54c285
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/456301
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Jesus de Vicente Pena <devicentepena@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47153}