NPE protection for purge requests (ONOS-1463)

Change-Id: I02732ed83aaaf5c760fdb96899696177415ebd23
diff --git a/core/net/src/main/java/org/onosproject/net/intent/impl/phase/IntentProcessPhase.java b/core/net/src/main/java/org/onosproject/net/intent/impl/phase/IntentProcessPhase.java
index 1ed42fd..399a307 100644
--- a/core/net/src/main/java/org/onosproject/net/intent/impl/phase/IntentProcessPhase.java
+++ b/core/net/src/main/java/org/onosproject/net/intent/impl/phase/IntentProcessPhase.java
@@ -50,7 +50,7 @@
             case WITHDRAW_REQ:
                 return new WithdrawRequest(processor, data, Optional.ofNullable(current));
             case PURGE_REQ:
-                return new PurgeRequest(data, current);
+                return new PurgeRequest(data, Optional.ofNullable(current));
             default:
                 // illegal state
                 return new Failed(data);