roll: third_party/pigweed pw_grpc: Add clear_connection to GrpcChannelOutput

Add a clear_connection() method to GrpcChannelOutput to allow resetting
the Connection reference. This is needed to prevent use-after-free
scenarios when the Connection object is destroyed but GrpcChannelOutput
retains a dangling reference to it.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/414672
Original-Revision: 170ef8c21e3b058e4a468aed8628f24cd512626c

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 77591bc8702a1b..170ef8c21e3b05
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8680552366198305697
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I9b5df690571802597dcb559429766c86a98dda6c
Reviewed-on: https://pigweed-review.googlesource.com/c/gonk/+/414792
diff --git a/third_party/pigweed b/third_party/pigweed
index 77591bc..170ef8c 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 77591bc8702a1bd37608ffaee9027265cf2f32e7
+Subproject commit 170ef8c21e3b058e4a468aed8628f24cd512626c