Backport bugfix for NPE to onos-1.5

(from commit 04b1fe9a)

Change-Id: Ie2af821e232bc8a1c9d6828c464bf3e54ad8cd11

@@ -343,7 +343,11 @@ public class DistributedDhcpStore implements DhcpStore {
 
 
     @Override
     @Override
     public IpAssignment getIpAssignmentFromAllocationMap(HostId hostId) {
     public IpAssignment getIpAssignmentFromAllocationMap(HostId hostId) {
-        return allocationMap.get(hostId).value();
+        if (allocationMap.get(hostId) != null) {
+            return allocationMap.get(hostId).value();
+        } else {
+            return null;
+        }
     }
     }
 
 
     /**
     /**