Guard malformed intent by pre-condition check on instantiation

Change-Id: I90ca0fac2e3b68c4ca5b464777b0cdaa2227c2e5
diff --git a/core/api/src/main/java/org/onlab/onos/net/intent/HostToHostIntent.java b/core/api/src/main/java/org/onlab/onos/net/intent/HostToHostIntent.java
index e09767a..b9baf10 100644
--- a/core/api/src/main/java/org/onlab/onos/net/intent/HostToHostIntent.java
+++ b/core/api/src/main/java/org/onlab/onos/net/intent/HostToHostIntent.java
@@ -71,6 +71,8 @@
         super(id(HostToHostIntent.class, min(one, two), max(one, two),
                  selector, treatment, constraints),
               appId, null, selector, treatment, constraints);
+
+        // TODO: consider whether the case one and two are same is allowed
         this.one = checkNotNull(one);
         this.two = checkNotNull(two);