Fix #12607: Improve admin directory check on login_page

The warning on login_page.php for the /admin/ directory would sometimes
not appear even if files within the /admin/ directory were accessible.

This check has been improved to be more pedantic and better suited to
cross-platform environments.

login_page.php

@@ -221,11 +221,12 @@
 
 	# Check if the admin directory is available and is readable.
 	$t_admin_dir = dirname( __FILE__ ) . DIRECTORY_SEPARATOR . 'admin' . DIRECTORY_SEPARATOR;
-	if ( is_dir( $t_admin_dir ) && is_executable( $t_admin_dir ) ) {
+	if ( is_dir( $t_admin_dir ) ) {
 		echo '<div class="warning" align="center">', "\n";
 		echo '<p><font color="red">', lang_get( 'warning_admin_directory_present' ), '</font></p>', "\n";
 		echo '</div>', "\n";
-
+	}
+	if ( is_dir( $t_admin_dir ) && is_readable( $t_admin_dir ) && is_executable( $t_admin_dir ) && @file_exists( "$t_admin_dir/." ) ) {
 		# since admin directory and db_upgrade lists are available check for missing db upgrades
 		# Check for db upgrade for versions < 1.0.0 using old upgrader
 		$t_db_version = config_get( 'database_version' , 0 );