Remove external ipv6 brackets if any in servername

Signed-off-by: Patrik Cyvoct <patrik@ptrk.io>

advanced/index.php

@@ -8,6 +8,8 @@
 
 // Sanitise HTTP_HOST output
 $serverName = htmlspecialchars($_SERVER["HTTP_HOST"]);
+// Remove external ipv6 brackets if any 
+$serverName = preg_replace('/^\[(.*)\]$/', '${1}', $serverName);
 
 if (!is_file("/etc/pihole/setupVars.conf"))
   die("[ERROR] File not found: <code>/etc/pihole/setupVars.conf</code>");