Implements a virtual destructor on the BaseRequestHandler class.

Implements a virtual destructor on the BaseRequestHandler class to protect against undefined behavior in
the event that a derived BaseRequestHandler class has a destructor and an object of such derived class
is destroyed through a pointer to its base class.

src/bitcoin-cli.cpp

@@ -198,6 +198,7 @@ static void http_error_cb(enum evhttp_request_error err, void *ctx)
 class BaseRequestHandler
 {
 public:
+    virtual ~BaseRequestHandler() {}
     virtual UniValue PrepareRequest(const std::string& method, const std::vector<std::string>& args) = 0;
     virtual UniValue ProcessReply(const UniValue &batch_in) = 0;
 };