NPE fix for ONOS-379

Change-Id: If928809b3f29c1e7e457c84ba229279137196b35

@@ -364,12 +364,13 @@ public class HazelcastLinkResourceStore
                 // concurrent allocation detected, retry transaction
                 throw new TransactionException("Concurrent Allocation, retry");
             }
-        }
+        } else {
             List<LinkResourceAllocations> after = new ArrayList<>(before.size() + 1);
             after.addAll(before);
             after.add(allocations);
             linkAllocs.replace(linkKey, before, after);
         }
+    }
 
     @Override
     public LinkResourceEvent releaseResources(LinkResourceAllocations allocations) {