fix: check code presence

Signed-off-by: m.nabokikh <maksim.nabokikh@flant.com>

server/handlers.go

@@ -805,6 +805,11 @@ func (s *Server) handleAuthCode(w http.ResponseWriter, r *http.Request, client s
 	code := r.PostFormValue("code")
 	redirectURI := r.PostFormValue("redirect_uri")
 
+	if code == "" {
+		s.tokenErrHelper(w, errInvalidRequest, `Required param: code.`, http.StatusBadRequest)
+		return
+	}
+
 	authCode, err := s.storage.GetAuthCode(code)
 	if err != nil || s.now().After(authCode.Expiry) || authCode.ClientID != client.ID {
 		if err != storage.ErrNotFound {