xerrors: fix Is panics if target is uncomparable

Analogous change to golang.org/cl/175260

Fixes golang/go#31841

Change-Id: I93bf093e8d5c13269fe7164a3a1b6956450998e5
Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/174980
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed