Backport bugfix for NPE to onos-1.5

(from commit 04b1fe9adab1382e89a743174b40505f900cd6b9)

Change-Id: Ie2af821e232bc8a1c9d6828c464bf3e54ad8cd11
diff --git a/apps/dhcp/app/src/main/java/org/onosproject/dhcp/impl/DistributedDhcpStore.java b/apps/dhcp/app/src/main/java/org/onosproject/dhcp/impl/DistributedDhcpStore.java
index 8c880a5..126cb8d 100644
--- a/apps/dhcp/app/src/main/java/org/onosproject/dhcp/impl/DistributedDhcpStore.java
+++ b/apps/dhcp/app/src/main/java/org/onosproject/dhcp/impl/DistributedDhcpStore.java
@@ -343,7 +343,11 @@
 
     @Override
     public IpAssignment getIpAssignmentFromAllocationMap(HostId hostId) {
-        return allocationMap.get(hostId).value();
+        if (allocationMap.get(hostId) != null) {
+            return allocationMap.get(hostId).value();
+        } else {
+            return null;
+        }
     }
 
     /**