Bug 20781: Set a minimum enrollemnt period 'in months' of 1

Bug 20781: Set a minimum enrollemnt period 'in months' of 1

To test:
 1 - Browse to Admin->Patron Categories
 2 - Click '+ New category'
 3 - Enter data as required, however, enter 0 in 'Enrolment period' ->
'in months'
 4 - Save
 5 - View the list of categories, the new category has 'Enrolment period'
= 'until'
 6 - Create a patron in this category, note their dateexipry is
'0000-00-00'
 7 - Search for them
 8 - internal server error
 9 - Delete that patron
10 - Apply patch
11 - Edit the category, note you cannot save with 0 months
12 - Set 1 month and save
13 - Create a patron in this cateogry
14 - Note they have a valid dateexpiry
15 - Search for them
16 - All works
17 - Create a new category and verify you cannot add with 0 months

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>