fmt: fix handling of %% verb in Scanf

fmt: fix handling of %% verb in Scanf

There were a couple of bugs, including not requiring a percent and
returning the wrong error for a bad format containing %%.

Both are addressed by fixing the first.

Fixes #34180.

Change-Id: If96c0c0258bcb95eec49871437d719cb9d399d9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/202879
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>