Bug 24658: (bug 22008 follow-up) Correct update DB entry 18.12.00.031

Bug 24658: (bug 22008 follow-up) Correct update DB entry 18.12.00.031

If the original constraint does not exist, the second one is not added.

Test plan:
1. checkout dbrev 18.12.00.030
2. Remove the constraint: ALTER TABLE accountlines DROP FOREIGN KEY accountlines_ibfk_2
3. checkout master
4.
% updatedatabase
=> The constraint accountlines_ibfk_items is not added
5. Repeat 1 to 4
=> The constraint is added correctly

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>