Fix x509 OpenSSL format when multiple OU's are present

When formatting DN in OpenSSL format Multiple Organizational Unit Names (OU) in certificates throw `PHP Notice:  Undefined index: id-at-organizationalUnitName` and returned array only contains the last OU.

phpseclib/File/X509.php

@@ -2977,7 +2977,7 @@ class File_X509
             }
             $output.= $desc . '=' . $value;
             $result[$desc] = isset($result[$desc]) ?
-                array_merge((array) $dn[$prop], array($value)) :
+                array_merge((array) $result[$desc], array($value)) :
                 $value;
             $start = false;
         }