xerrors: fix crash in As Analogous change to CL 168598 Fixes golang/go#30970. Change-Id: I1810418a9d7cf92a35ca0f4a6d1b2bad560ab671 Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/170960 Reviewed-by: Damien Neil <dneil@google.com> Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org>