add specific chars for matching of ipv6 addresses (#1040)

when validating servers in the `Host` property.

class.phpmailer.php

@@ -1622,7 +1622,7 @@ class PHPMailer
 
         foreach ($hosts as $hostentry) {
             $hostinfo = array();
-            if (!preg_match('/^((ssl|tls):\/\/)*([a-zA-Z0-9\.-]*):?([0-9]*)$/', trim($hostentry), $hostinfo)) {
+            if (!preg_match('/^((ssl|tls):\/\/)*([a-zA-Z0-9:\[\]\.-]*):?([0-9]*)$/', trim($hostentry), $hostinfo)) {
                 // Not a valid host entry
                 continue;
             }