update.php gives warning when there are no updates

if you run update.php in HEAD when there are no updates available, you get the following:

warning: reset(): Passed variable is not an array or object in /Users/wright/drupal/cvs/head/core.cvs/update.php on line 424.

attached is a small patch to fix this.

potentially, we should do something much smarter if the user clicks "update" when there are no available updates (e.g. not hit the entire progress bar code path, the JS, etc, etc, and just immediately go to the finished page with a message about "no updates were performed"). however, that would involve a much bigger patch, and i wanted to keep this small.