[roll third_party/pigweed] pw_transfer: Properly clean up transfer_thread

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/92560

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 4a217c7500f074b..faf42d6fe040985
Roller-URL: https://ci.chromium.org/b/8813614778236861185
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8813614778236861185
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I64ccbaf4d2cdfdb31b9d317ed3b63f3aac5326a5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/95444
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>
1 file changed