internal/zstd: handle match extending past window
For #62513
Change-Id: I59c24b254d5073140811b41497eabb91fb0046e9
GitHub-Last-Rev:
4dd16fcfa813da2b612d5753e11c163476d44b53
GitHub-Pull-Request: golang/go#63248
Reviewed-on: https://go-review.googlesource.com/c/go/+/531255
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>