OperatorConcat - use BackpressureUtils to prevent request overflow and add n > 0 check to prevent race condition

Author: davidmoten

src/main/java/rx/internal/operators/OperatorConcat.java

@@ -115,7 +115,7 @@ public void onStart() {
         private void requestFromChild(long n) {
             // we track 'requested' so we know whether we should subscribe the next or not
             ConcatInnerSubscriber<T> actualSubscriber = currentSubscriber;
-            if (REQUESTED_UPDATER.getAndAdd(this, n) == 0) {
+            if (n > 0 && BackpressureUtils.getAndAddRequest(REQUESTED_UPDATER, this, n) == 0) {
                 if (actualSubscriber == null && wip > 0) {
                     // this means we may be moving from one subscriber to another after having stopped processing
                     // so need to kick off the subscribe via this request notification