There's no guarantee that all in-progress mark assists will finish
before the trace does. Don't crash if that happens.

I haven't added a test because there's quite a bit of ceremony involved
and the bug is fairly straightforward.

Change-Id: Ia1369a8e2260fc6a328ad204a1eab1063d2e2c90
Reviewed-on: https://go-review.googlesource.com/37540Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>