adjacency-list: fix crash in equals operator

some minor code improvements

refs: #4705

Change-Id: I28805981a88a94c79f699956bb7e62a268d9ba49
diff --git a/src/route/fib.hpp b/src/route/fib.hpp
index 0c561ab..27e5967 100644
--- a/src/route/fib.hpp
+++ b/src/route/fib.hpp
@@ -89,7 +89,7 @@
    * \param allHops A complete list of next-hops to associate with name.
    */
   VIRTUAL_WITH_TESTS void
-  update(const ndn::Name& name, NexthopList& allHops);
+  update(const ndn::Name& name, const NexthopList& allHops);
 
   /*! \brief Remove all entries from the FIB.
    *
@@ -160,10 +160,10 @@
    * \sa Fib::removeOldNextHopsFromFibEntryAndNfd
    */
   void
-  addNextHopsToFibEntryAndNfd(FibEntry& entry, NexthopList& hopsToAdd);
+  addNextHopsToFibEntryAndNfd(FibEntry& entry, const NexthopList& hopsToAdd);
 
   unsigned int
-  getNumberOfFacesForName(NexthopList& nextHopList);
+  getNumberOfFacesForName(const NexthopList& nextHopList);
 
   /*! \brief Unregisters a prefix from NFD's RIB.
    *