Formatting keys in labels can panic when a label is constructed with a nil
error. Avoid that by not passing the defective label to event.Log.
Change-Id: I3099a7ac48c5830af1072141f2b619d0e0fbcf5a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/229985
Run-TryBot: Peter Weinberger <pjw@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>