Fix NPE when there's no secondary path.

Change-Id: I3bb8eeb2abf140d44cf4dc7186cf1bf316c23f70
diff --git a/utils/misc/src/main/java/org/onlab/graph/DisjointPathPair.java b/utils/misc/src/main/java/org/onlab/graph/DisjointPathPair.java
index 0e25116..1e8d3d6 100644
--- a/utils/misc/src/main/java/org/onlab/graph/DisjointPathPair.java
+++ b/utils/misc/src/main/java/org/onlab/graph/DisjointPathPair.java
@@ -66,7 +66,7 @@
     /**
      * 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;