bug 1684 fixed: the test to check availability of the user_infos line was
wrong. I had changed the old db_num_rows > 0 because it was not working with
SQLite. As suggested by nicolas, let's use a simpler trick "count(1)" in the
query itself, this way it should work with any database engine.
I've also removed the while (true) (ugly infinite loop, with a condition for
exit) that was producing an infinite loop for Piwigo installations with 2.0
database model and 2.1 code (before launching upgrade.php)