runtime: fix mishandling of timer zombie count

runtime: fix mishandling of timer zombie count

The timer zombie count was fundamentally racy and worked around
in CL 569995. We worked around that by ignoring underflow.
The fundamnental race was because t.ts was set before t was
inserted into ts. CL 564997 corrected that fundamental problem,
so now we can account for zombies completely accurately,
never seeing values less than zero. Do that.

Change-Id: Idfbccc6662af5935f29f2a06a35e8ea93929bed7
Reviewed-on: https://go-review.googlesource.com/c/go/+/569996
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Austin Clements <austin@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>