[roll third_party/pigweed] pw_tokenizer: Display as prefixed Base64 if decoding fails

If a tokenized message cannot be decoded, use the prefixed Base64
version for __str__ rather than an empty string. This makes it simple
to decode the string later.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/38260

third_party/pigweed Rolled-Commits: 8592008642e8675..7df6664d5739fa1
Roller-URL: https://ci.chromium.org/b/8852485806524403328
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8852485806524403328
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I60ffac4f6b9274d2401c7f4525ea648e32b1b958
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/38320
Reviewed-by: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 8592008..7df6664 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 8592008642e8675a0360c673e632441b80be789c
+Subproject commit 7df6664d5739fa101c041ff726c1f0345ec0fbec