[roll third_party/pigweed] pw_multisink: Fix compiler warnings for tests

GCC warns about constructing a MultiBuf with an uninitialized array, so
initialize it first. This isn't a problem in practice because the
uninitialized data is never read.

Also, ensure the test builds when joining is disabled, though none of
the tests can run.

Original-Bug: b/343480404
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/212051
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https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 694805ff7c3e249..272873750faa652
Roller-URL: https://ci.chromium.org/b/8746475709021352449
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Icc443dc3357a88bfd9bf0c15929b499893a31240
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/quickstart/bazel/+/212416
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Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 694805f..2728737 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 694805ff7c3e249a9e5a380557f16493e3baa40c
+Subproject commit 272873750faa65275e9e1a0340289c2992692cfc