runtime: don't clear timerModifiedEarliest if adjustTimers is 0

This avoids a race when a new timerModifiedEarlier timer is created by
a different goroutine.

Fixes #47329

Change-Id: I6f6c87b4a9b5491b201c725c10bc98e23e0ed9d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/336432


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>