virtual destructors and missing @end in tutorial

doc/tutorial/tracing.texi

@@ -2255,7 +2255,7 @@ implementation method.  For example, the lowest level pcap method,
 
 @verbatim
   void EnablePcap (std::string prefix, Ptr<NetDevice> nd, bool promiscuous = false);
-@verbatim
+@end verbatim
 
 will call the device implementation of @code{EnablePcapInternal} directly.  All
 other public pcap tracing methods build on this implementation to provide 

src/helper/csma-helper.h

@@ -50,6 +50,7 @@ public:
    * Construct a CsmaHelper.
    */
   CsmaHelper ();
+  virtual ~CsmaHelper () {}
 
   /**
    * \param type the type of queue

src/helper/point-to-point-helper.h

@@ -50,6 +50,7 @@ public:
    * point networks.
    */
   PointToPointHelper ();
+  virtual ~PointToPointHelper () {}
 
   /**
    * Each point to point net device must have a queue to pass packets through.