Read the latest value of lastSink

Multiple threads could access the field, lastSink, then they should read
the latest value of lastSink

Change-Id: I279ed6fa3db357008603cde5cb2c57671e66b33c
diff --git a/core/net/src/main/java/org/onosproject/event/impl/CoreEventDispatcher.java b/core/net/src/main/java/org/onosproject/event/impl/CoreEventDispatcher.java
index 26a89d4..e44709c 100644
--- a/core/net/src/main/java/org/onosproject/event/impl/CoreEventDispatcher.java
+++ b/core/net/src/main/java/org/onosproject/event/impl/CoreEventDispatcher.java
@@ -73,7 +73,7 @@
 
     // Means to detect long-running sinks
     private TimerTask watchdog;
-    private EventSink lastSink;
+    private volatile EventSink lastSink;
     private final Stopwatch stopwatch = Stopwatch.createUnstarted();
     private volatile Future<?> dispatchFuture;