[roll third_party/pigweed] pw_multibuf: AdvanceToData in iterator constructor

- Fix iteration over MultiBufs that start with an empty chunk.
- Make non-default iterator construction private to MultiBuf.
- Remove the unnecessary byte_index iterator constructor parameter.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/214503
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https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 98c249943f7a441..9ea9ef130481b82
Roller-URL: https://ci.chromium.org/b/8745845099641202273
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Change-Id: Id9de8e82ca47d87082786b2dc020e229ca88f95b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/quickstart/bazel/+/214515
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
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diff --git a/third_party/pigweed b/third_party/pigweed
index 98c2499..9ea9ef1 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 98c249943f7a44166db0b60eda88d0acbd3d2e0e
+Subproject commit 9ea9ef130481b8287e7c0d1432dd1544f79bb561