I can't remember just how this happened to me, but I got an unfortunate
crash with some set of cmd/compile debug options and source code.

Change-Id: Ibef6129c50b68dad0594ac439466bfbc4b32a095
Reviewed-on: https://go-review.googlesource.com/19920
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>