In my experience, this usually happens when vet panics.
Dumping all unparseable lines should help diagnosis.

Inspired by the trybot failures in CL 40511.

Change-Id: Ib73e8c8b2942832589c3cc5d33ef35fdafe9965a
Reviewed-on: https://go-review.googlesource.com/40508Reviewed-by: Rob Pike <r@golang.org>