Update whenComplete to include a synchronization lock

Author: RanVaknin

services-custom/s3-transfer-manager/src/main/java/software/amazon/awssdk/transfer/s3/internal/AsyncBufferingSubscriber.java

@@ -56,8 +56,10 @@ public AsyncBufferingSubscriber(Function<T, CompletableFuture<?>> consumer,
         returnFuture.whenComplete((r, t) -> {
             if (t != null) {
                 requestsInFlight.forEach(f -> f.cancel(true));
-                if (subscription != null) {
-                    subscription.cancel();
+                synchronized (this) {
+                    if (subscription != null) {
+                        subscription.cancel();
+                    }
                 }
             }
         });