synchronize access to subscription for null check in OperatorMulticast

Author: davidmoten

rxjava-core/src/main/java/rx/internal/operators/OperatorMulticast.java

@@ -137,8 +137,13 @@ public void call() {
             }));
 
             // now that everything is hooked up let's subscribe
-            if (subscription!=null)
-               source.unsafeSubscribe(subscription);
+            // as long as the subscription is not null
+            boolean subscriptionIsNull;
+            synchronized(guard) {
+                subscriptionIsNull = subscription == null;
+            }
+            if (!subscriptionIsNull)
+                source.unsafeSubscribe(subscription);
         }
     }
 }