Func `Wait` in package ratelimit could got blocked if time moves backwards for any reason

[lichuqiang]

We import package ratelimit in Kubernetes and met a scenario, not sure whether the issue should be filed here.
The scenario is that when time happened moves backwards for any reason, func take that calculate the time to sleep would get a huge number here:

	endTick := currentTick + (-avail+tb.quantum-1)/tb.quantum
	endTime := tb.startTime.Add(time.Duration(endTick) * tb.fillInterval)
	waitTime := endTime.Sub(now)
	if waitTime > maxWait {
		return 0, false
	}

An available way for the issue might be adding a protection before calculating waitTime, say a comparision between now and startTime.

/cc @thockin

[rogpeppe]

The scenario is that when time happened moves backwards for any reason, func take that calculate the time to sleep would get a huge number here:

Given that time.Duration is signed, won't that happen only if the time moves back more than ~290 years?
I guess that it's possible that might happen in endTime is the zero time. Is that what happened?

Checking for overflow seems like a reasonable plan anyway though - PRs happily accepted. It would be good if it didn't slow things down much.