Check if Nextcloud is installed

Fixes https://github.com/nextcloud/server/issues/4735



Signed-off-by: Lukas Reschke <lukas@statuscode.ch>

core/templates/layout.guest.php

@@ -40,7 +40,8 @@
 								<h1 class="hidden-visually">
 									<?php p($theme->getName()); ?>
 								</h1>
-								<?php if(\OC::$server->getConfig()->getAppValue('theming', 'logoMime', false)): ?>
+								<?php if(\OC::$server->getConfig()->getSystemValue('installed', false)
+									&& \OC::$server->getConfig()->getAppValue('theming', 'logoMime', false)): ?>
 									<img src="<?php p($theme->getLogo()); ?>"/>
 								<?php endif; ?>
 							</div>