Restoring null check removed in recent commit

See ReactiveX@caf3a94 for removal

It started causing this unit test to fail:

rx.operators.OperationZip$UnitTest > testAggregatorUnsubscribe FAILED
    java.lang.NullPointerException at OperationZip.java:611

Not sure how I missed it when I reviewed the previous commit … but I did.

Author: benjchristensen

rxjava-core/src/main/java/rx/operators/OperationZip.java

@@ -259,8 +259,10 @@ private void stop() {
             /* tell ourselves to stop processing onNext events by setting running=false */
             if (running.compareAndSet(true, false)) {
                 /* propogate to all Observers to unsubscribe if this thread succeeded in setting running=false */
-                for (ZipObserver<T, ?> rw : observers) {
-                    rw.subscription.unsubscribe();
+                for (ZipObserver<T, ?> o : observers) {
+                    if (o.subscription != null) {
+                        o.subscription.unsubscribe();
+                    }
                 }
             }
         }