table: ensure NameTree::lookup(tableEntry) does not return nullptr

NameTree::lookup(tableEntry) is always equivalent to NameTree::lookup(Name).
NameTree::getEntry(tableEntry) should be used when inserting new name tree entry is undesired.

refs #3687

Change-Id: I0b17cd07dde83341976cfe66c85855dfb2fa6e89
diff --git a/daemon/table/fib.hpp b/daemon/table/fib.hpp
index 950f238..0a44166 100644
--- a/daemon/table/fib.hpp
+++ b/daemon/table/fib.hpp
@@ -148,7 +148,7 @@
 
 private:
   const Entry&
-  findLongestPrefixMatch(const name_tree::Entry* nte) const;
+  findLongestPrefixMatch(const name_tree::Entry& nte) const;
 
   void
   erase(name_tree::Entry* nte, bool canDeleteNte = true);