Auth: don't call callback twice.

src/IRequest.h

@@ -226,6 +226,8 @@ class GenericCallback {
   GenericCallback(typename IGenericCallback<Arguments...>::Pointer functor)
       : functor_(functor) {}
 
+  GenericCallback(std::nullptr_t) {}
+
   operator bool() const { return static_cast<bool>(functor_); }
 
   void operator()(Arguments... d) const { functor_->done(d...); }

src/Utility/Auth.cpp

@@ -47,7 +47,7 @@ IHttpServer::IResponse::Pointer Auth::HttpServerCallback::handle(
   const char* error = request.get(data_.error_parameter_name_);
   if (accepted) {
     std::unique_lock<std::mutex> lock(lock_);
-    auto callback = std::move(data_.callback_);
+    auto callback = util::exchange(data_.callback_, nullptr);
     lock.unlock();
     if (callback) {
       if (std::string(accepted) == "true" && code) {