Fix the bug to be able to log in with any password when migrating from phpBB3

"Password Converter" has this problem and this commit fixes it.

There is a condition for checking phpBB3 pasword. This condition is a lack of
blackets and cause $authorized to be true when $cur_usr don't has the value of
'salt'.

This results in the success of logging in with any password as long as the
account name exists in database.