[#30815] Check updates for extensions causes php errors

Fixed the following problem.
When browsing the Administrator dashboard:

PHP Notice:  Undefined property: JUpdaterExtension::$currentUpdate in libraries/joomla/updater/adapters/extension.php on line 97

PHP Notice:  Trying to get property of non-object in libraries/joomla/updater/adapters/extension.php on line 97

The two problems are on the same line, but they are due to different reasons.