adjacency-list: fix crash in equals operator

some minor code improvements

refs: #4705

Change-Id: I28805981a88a94c79f699956bb7e62a268d9ba49
diff --git a/src/route/name-prefix-table-entry.hpp b/src/route/name-prefix-table-entry.hpp
index b345231..828d64a 100644
--- a/src/route/name-prefix-table-entry.hpp
+++ b/src/route/name-prefix-table-entry.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2014-2017,  The University of Memphis,
+ * Copyright (c) 2014-2018,  The University of Memphis,
  *                           Regents of the University of California,
  *                           Arizona Board of Regents.
  *
@@ -75,8 +75,8 @@
     return m_rteList.size();
   }
 
-  NexthopList&
-  getNexthopList()
+  const NexthopList&
+  getNexthopList() const
   {
     return m_nexthopList;
   }