os: correctly handle errno==0 in (*Process).blockUntilWaitable
CL 627478 inadvertently returns a non-nil error
containing a syscall.Errno(0).
Change-Id: I1d6a9d0575d3ed651ddc02f30505437d0d266bb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/629515
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>