Bug 15746: Do not record individual payments with randomly picked libraries

Bug 15746: Do not record individual payments with randomly picked libraries

When a payment is made individualy, the library used to record this
payment is randomly picked from the library list.
This is because C4::Branch::GetBranch looks at 1. the $branch variable
sent by the template, wich does not exist, then 2. the branchname
cookie, which does not exist neither, then 3. get the first branchcode
from a list of keys (non ordered).

To reproduce:
- Create a manual invoice for a patron
(members/maninvoice.pl?borrowernumber=XXXX)
- Pay this fine using the "Pay" button of the corresponding line from
  the members/pay.pl?borrowernumber=XXXX page
Look at the statistics table:
select * from statistics order by datetime desc limit 10;

The branch value of the first line might not correspond to the library
you were using to pay the payment.

Test plan:
Apply this patch, repeat the steps above and confirm that the library
picked is now the one used to pay.

Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
Works as advertised

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com