Fix bug in access_has_bug_level() for private issues

When private_bug_threshold is defined as an array instead of a single
access level, e.g. array(0=>40, 1=>70, 2=>90) to prevent developers
from seeing private bugs while granting that privilege to updaters,
access_has_bug_level() incorrectly returned true.

The consequence is that unwanted access to Private bugs was granted to
users who are allowed to view them, e.g. allowing them to delete or
perform other restricted actions.

Fixes 0010124