allow null Master in MastershipTerm

Change-Id: I840354eb6d0b5a1bac91887a41626c33c49d592c
diff --git a/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java b/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
index d1dab70..178a1a3 100644
--- a/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
+++ b/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
@@ -257,7 +257,8 @@
             MastershipTerm term = mastershipService.requestTermService()
                     .getMastershipTerm(deviceId);
 
-            if (!term.master().equals(clusterService.getLocalNode().id())) {
+            final NodeId myNodeId = clusterService.getLocalNode().id();
+            if (!myNodeId.equals(term.master())) {
                 // lost mastership after requestRole told this instance was MASTER.
                 log.info("lost mastership before getting term info.");
                 return;