Add virtual default DTOR to ClientEvent::Base

ClientEvent::Base is the base class for many other classes including
a few that add data members. If at some point one of these enhanced
derived classes is referenced and then deleted via a base class
pointer, some memory could leak.

I don't think we do that yet, but it seems worth preventing.

Signed-off-by: Charlie Vigue <charlie.vigue@openvpn.com>

openvpn/client/clievent.hpp

@@ -169,6 +169,7 @@ class Base : public RC<thread_safe_refcount>
 {
  public:
  typedef RCPtr<Base> Ptr;
+ virtual ~Base() = default;
  Base(Type id)
  : id_(id)
  {