MDL-64223 libraries: PHP7.1 compatibility fixes for Jabber lib

lib/jabber/XMPP/XMLStream.php

@@ -126,11 +126,11 @@ class XMPPHP_XMLStream {
 	/**
 	 * @var string
 	 */
-	protected $until = '';
+	protected $until = [];
 	/**
 	 * @var string
 	 */
-	protected $until_count = '';
+	protected $until_count = [];
 	/**
 	 * @var array
 	 */

lib/jabber/readme_moodle.txt

@@ -2,3 +2,4 @@ Description of XMPPHP (aka jabber) version 0.1rc2-r77 library import into Moodle
 
 MDL-20876 - replaced deprecated split() with explode()
 MDL-52335 - PHP7 variable syntax changes
+MDL-64223 - PHP7.1 compatibility changes