making CN with a , inside working

a workaround to mask a , sign in a DN before CN is extracted
resolves #3533

html/includes/authentication/active_directory.inc.php

@@ -295,7 +295,9 @@ function get_dn($samaccountname) {
 }
 
 function get_cn($dn) {
+    dn = str_replace('\\,','~',$dn);
     preg_match('/[^,]*/', $dn, $matches, PREG_OFFSET_CAPTURE, 3);
+    $matches[0][0] = str_replace('~',',',$matches[0][0]);
     return $matches[0][0];
 }