Fix GossipIntentStore purge logic to ignore purge requests for non-existent intent data

Change-Id: Ie5083ca7096bcdf1f94e80fe88d9ef65ac0f2420

@@ -176,7 +176,7 @@ public class GossipIntentStore
         if (IntentData.isUpdateAcceptable(currentData, newData)) {
             // Only the master is modifying the current state. Therefore assume
             // this always succeeds
-            if (newData.state() == PURGE_REQ) {
+            if (newData.state() == PURGE_REQ && currentData != null) {
                 currentMap.remove(newData.key(), currentData);
             } else {
                 currentMap.put(newData.key(), new IntentData(newData));