fixes #862 - Piwik_Tracker.disconnectDatabase should set self::$db to null; patch from ckoulman

git-svn-id: http://dev.piwik.org/svn/trunk@1304 59fd770c-687e-43c8-a1e3-f5a4ff64c105

core/Tracker.php

@@ -177,6 +177,7 @@ class Piwik_Tracker
 		if(isset(self::$db))
 		{
 			self::$db->disconnect();
+			self::$db = null;
 		}
 	}