RequestServer: Fix assertion on exit when curl had pending timeouts

If we already destroyed our timer during destruction, and then curl
tries to flush its timeouts when we tear down the multi, we can just
ignore the timer callbacks.

Userland/Services/RequestServer/ConnectionFromClient.cpp

@@ -163,6 +163,8 @@ int ConnectionFromClient::on_socket_callback(CURL*, int sockfd, int what, void*
 int ConnectionFromClient::on_timeout_callback(void*, long timeout_ms, void* user_data)
 {
     auto* client = static_cast<ConnectionFromClient*>(user_data);
+    if (!client->m_timer)
+        return 0;
     if (timeout_ms < 0) {
         client->m_timer->stop();
     } else {