Fix client crashing when connecting to server

My commit

e2d54c9f "shutdown when requested from lua in singleplayer too"

broke minetest's feature to connect to servers. The client crashed
after the connection init was complete.

Thanks to @sofar for reporting the bug.

Fixes #3498.

src/game.cpp

@@ -1818,7 +1818,7 @@ void Game::run()
 
 	while (device->run()
 			&& !(*kill || g_gamecallback->shutdown_requested
-			|| server->getShutdownRequested())) {
+			|| (server && server->getShutdownRequested()))) {
 
 		/* Must be called immediately after a device->run() call because it
 		 * uses device->getTimer()->getTime()