MDL-35344 Ignore available updates info with invalid format
Previously when the server response validation had failed, the property
available_update_checker::recentresponse was left as NULL. This caused
problems in available_update_checker::compare_responses() that expects
proper arrays passed.
This patch makes invalid data being ignored by setting the property to
an empty array as if no data have been fetched yet. A debugging message
is thrown so the admins will be able to diagnose eventual problems in
the future.
