Fix generic error when anonymous login not defined

When $g_allow_anonymous_login = ON and $g_anonymous_account = '', a
Generic error is triggered in auth_flags() when trying to login
anonymously. This is due to the fact that $p_user_id parameter is false
in this case.

To prevent this, the function now performs a loose-type check on the
user id, so MantisBT returns to the login page with a friendlier error
message "Your account may be disabled or blocked or the
username/password you entered is incorrect."

Fixes 0025061