[roll third_party/pigweed] pw_transfer: Fix start logic

Don't set a transfer context's handler (activating the transfer) if its
Prepare function fails.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/55509

third_party/pigweed Rolled-Commits: 5fbfd88c5885f19..32a875ddb038207
Roller-URL: https://ci.chromium.org/b/8839963411637771456
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8839963411637771456
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I4a18c6f897d46fa39163eaa8d820db7e3e2e6195
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/55842
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 5fbfd88..32a875d 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 5fbfd88c5885f1999f2c1dd4aecbcc2c5df47f20
+Subproject commit 32a875ddb038207df964d93028acfd248a7a9007