Fix NPE when there's no secondary path.

Change-Id: I3bb8eeb2abf140d44cf4dc7186cf1bf316c23f70

@@ -43,7 +43,7 @@ public interface DisjointPath extends Path {
     /**
      * Gets secondary path.
      *
-     * @return secondary path
+     * @return secondary path, or null if there is no secondary path available.
      */
     Path backup();
 }

@@ -482,6 +482,12 @@ public class DefaultTopology extends AbstractModel implements Topology {
     }
 
     private DisjointPath networkDisjointPath(DisjointPathPair<TopologyVertex, TopologyEdge> path) {
+        if (!path.hasBackup()) {
+            // There was no secondary path available.
+            return new DefaultDisjointPath(CORE_PROVIDER_ID,
+                                           (DefaultPath) networkPath(path.primary()),
+                                           null);
+        }
         return new DefaultDisjointPath(CORE_PROVIDER_ID,
                                        (DefaultPath) networkPath(path.primary()),
                                        (DefaultPath) networkPath(path.secondary()));

@@ -66,7 +66,7 @@ public class DisjointPathPair<V extends Vertex, E extends Edge<V>> implements Pa
     /**
      * Returns the secondary path.
      *
-     * @return primary path
+     * @return secondary path, or null if there is no secondary path available.
      */
     public Path<V, E> secondary() {
         return secondary;