Subscriber.request add ignore for negative requests

[davidmoten]

So that negative requests don't need to be checked by every Operator that supports backpressure why don't we add a check at the start of Subscriber.request:

protected final void request(long n) {
        // ignore zero or negative requests
        if (n <=0) return;
        Producer shouldRequest = null;
        synchronized (this) {
            if (p != null) {
                shouldRequest = p;
            } else {
                requested = n;
            }
        }
        // after releasing lock
        if (shouldRequest != null) {
            shouldRequest.request(n);
        }
    }

Happy to submit PR if has support.

[zsxwing]

Looks a good place to add it. Just a bit comment: Ignore zero but IAE for negative numbers, as per #1956

[davidmoten]

I did wonder about that, and makes sense. I'll make a PR.

[davidmoten]

Looks to me that n = 0 should be allowed through because it may override a default of Long.MAX_VALUE sometimes (asking an upstream producer to produce nothing as opposed to whatever a default might be). I'll make a PR with the n < 0 check only.