LinkDiscovery: check if Stopped before starting scheduled task.

Change-Id: I6c967525e61d1942efbe6aa221f89f9adebbd464
diff --git a/providers/lldp/src/main/java/org/onlab/onos/provider/lldp/impl/LinkDiscovery.java b/providers/lldp/src/main/java/org/onlab/onos/provider/lldp/impl/LinkDiscovery.java
index db52454..af600ac 100644
--- a/providers/lldp/src/main/java/org/onlab/onos/provider/lldp/impl/LinkDiscovery.java
+++ b/providers/lldp/src/main/java/org/onlab/onos/provider/lldp/impl/LinkDiscovery.java
@@ -241,6 +241,9 @@
      */
     @Override
     public void run(final Timeout t) {
+        if (isStopped()) {
+            return;
+        }
         boolean isMaster = mastershipService.getLocalRole(device.id()) == MASTER;
         if (!isMaster) {
             if (!isStopped()) {