Bug 30868: (bug 29684 follow-up) Fix borrowernumber var in template

Bug 30868: (bug 29684 follow-up) Fix borrowernumber var in template

borrower_data does not contain the borrowernumber, it is sent
separately.

Test plan:
1. Edit a patron record (for example: Henry Acevedo).
2. Note that the URL is http://127.0.0.1:8081/cgi-bin/koha/members/memberentry.pl?op=modify&destination=circ&borrowernumber=19
3. Change date of birth to yesterday.
4. Save.
5. Error message displayed above the Save and Cancel buttons:
   The following fields are wrong. Please fix them.
   - Patron's age is incorrect for their category. Ages allowed are 18-999.
6. Note that the URL is now http://127.0.0.1:8081/cgi-bin/koha/members/memberentry.pl
7. Fix date of birth (for example: change year to 2000).
8. Select Save.
=> Without this patch you get: "Patron not found. Return to search."
=> With this patch applied the patron's data is saved

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>