Bug 14399: Prevent display of double zero notforloan status

Bug 14399: Prevent display of double zero notforloan status

On bug 12913 a zero status is added on the inventory form. This prevents
a lot of false warnings for a wrong notforloan status. The zero status
is not included in the default setup. But if you would add one, the
status will display here twice now.

This patch checks if the status already exists.

Test plan:
[1] Add a zero NOTFORLOAN status if it does not yet exist in Authorized
    values.
[2] Check that you do not see two "For loan" statuses on inventory form.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>