pw_rpc: Guard packets vector usage in FakeServer with Output mutex
pw_rpc: Guard packets vector usage in FakeServer with Output mutex

FakeServer::SendResponse uses FakeServer::ProcessPacket under the hood.
Usually SendResponse is used when client side is also active.

ProcessPacket accesses packets_ vector from Output, but doesn't guard
its usage with the mutex. This CL adds a guard around the packets_usage
from the ProcessPacket() method.

Change-Id: Id726e028b18b8d50906e14c7b1fddfc1ed086037
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/97562
Pigweed-Auto-Submit: Dennis Kormalev <6222e5e705620133a24041cba003e993480020a9@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed