Avoid symlink() error if acme conf already exists as a file.

install/lib/installer_base.lib.php

@@ -2875,7 +2875,7 @@ class installer_base {
 		if(@is_link($vhost_conf_enabled_dir.'/' . $use_symlink)) {
 			unlink($vhost_conf_enabled_dir.'/' . $use_symlink);
 		}
-		if(!@is_link($vhost_conf_enabled_dir.'/' . $use_symlink)) {
+		if(!@is_file($vhost_conf_enabled_dir.'/' . $use_symlink)) {
 			symlink($vhost_conf_dir.'/' . $use_name, $vhost_conf_enabled_dir.'/' . $use_symlink);
 		}
 	}