cmd/go: fix infinite loop in modload.keepSums

Fixes #42891

Change-Id: I0cce4204a1c4959b896188a2ab3719c0507f95e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/274172


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>