NPE in dhcpl2rrealy when device mastership/availability changes.

Change-Id: Ibd933237cc264aca0a862c2620f8ac743a809e9c
diff --git a/src/main/java/org/opencord/dhcpl2relay/DhcpL2Relay.java b/src/main/java/org/opencord/dhcpl2relay/DhcpL2Relay.java
index 06a7f4f..ec4a726 100755
--- a/src/main/java/org/opencord/dhcpl2relay/DhcpL2Relay.java
+++ b/src/main/java/org/opencord/dhcpl2relay/DhcpL2Relay.java
@@ -188,7 +188,8 @@
         cancelDhcpPackets();
 
         componentConfigService.unregisterProperties(getClass(), false);
-
+        deviceService.removeListener(deviceListener);
+        mastershipService.removeListener(changeListener);
         log.info("DHCP-L2-RELAY Stopped");
     }