[roll third_party/pigweed] pw_rpc: Guard the channels list with the RPC mutex

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/80000

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: c4e9c62a8e6b36d..ddfd6e92fdd02c5
Roller-URL: https://ci.chromium.org/b/8824489261502384961
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8824489261502384961
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I571ab88dc73b6cc5df2c73ccabbede1b2293b457
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/80160
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index c4e9c62..ddfd6e9 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit c4e9c62a8e6b36d0deac9fea6bfb85059bedadd0
+Subproject commit ddfd6e92fdd02c58d78411ace55cfae07a4f8586