scanEnd is delayed one byte so we decrement
the scanner bytes count by 1 to ensure that
this value is correct in the next call of Decode.

Fixes #32399

Change-Id: I8c8698e7f95bbcf0373aceaa05319819eae9d86f
GitHub-Last-Rev: 0ac25d8de23d38c7ac577faddc6983571023f561
GitHub-Pull-Request: golang/go#32598
Reviewed-on: https://go-review.googlesource.com/c/go/+/182117Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>