[roll third_party/pigweed] pw_rpc: Conversion warning cleanups

This turns on -Wconversion for tests for pw_rpc, pw_protobuf, pw_varint,
pw_hdlc and pw_string, and fixes the issues that came up.

Original-Bug: b/259746255
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/146793

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 11fa22b210e7a0a..d6565261f5a55c9
Roller-URL: https://ci.chromium.org/b/8779005929546926001
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Iba7087f5f42d9cd58759bb96eb6b9442362c524b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/150826
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 11fa22b..d656526 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 11fa22b210e7a0a24893ab949914971da33b45bc
+Subproject commit d6565261f5a55c9028da7a37e6d458ab177503af