Issue #17099 shows a failure that indicates we rescanned a stack twice
concurrently during mark termination, which suggests that the rescan
list became inconsistent. Add a simple check when we dequeue something
from the rescan list that it claims to be at the index where we found
it.

Change-Id: I6a267da4154a2e7b7d430cb4056e6bae978eaf62
Reviewed-on: https://go-review.googlesource.com/29280
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>