This removes a concurrent access of ctxt.Data.

Updates #15756

Change-Id: Id017e90e47e093cd8825907f3853bb3d3bf8280d
Reviewed-on: https://go-review.googlesource.com/40507
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>