When this happens, panic.
This is a revised version of a check that used #next,
where this one instead uses a per-loop #exit flag,
and catches more problematic iterators.
Updates #56413.
Updates #61405.
Change-Id: I6574f754e475bb67b9236b4f6c25979089f9b629
Reviewed-on: https://go-review.googlesource.com/c/go/+/540263
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
