[roll third_party/pigweed] pw_multibuf: Pass reference instead of pointer

Since the allocator cannot be nullptr (it is immediately dereferenced),
take a reference instead.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/197132

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 13ebfcbcdbe52cf..61eafe32b059946
Roller-URL: https://ci.chromium.org/b/8753323563968732945
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I5ec6d4b8fc4679d8f53060198ffda3c79023b3cd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/197190
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 13ebfcb..61eafe3 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 13ebfcbcdbe52cf87ed7f95e2a717da9fc2afc01
+Subproject commit 61eafe32b0599469e6ab031a032acd27b3e70b4e