Bug 27604: Make sure PatronSelfRegistrationLibraryList is used

Bug 27604: Make sure PatronSelfRegistrationLibraryList is used

The value of PatronSelfRegistrationLibraryList is used to display the dropdown
list of the libraries, but not after the form is submitted.

Test plan:
1. Empty PatronSelfRegistrationLibraryList
2. Self register a new patron
=> Confirm the dropdown list with the libraries contain all the
libraries
3. Fill in PatronSelfRegistrationLibraryList with branchcodes (eg. "CPL|MPL")
4. Self register a new patron
5. Edit the HTML form and replace the option's value with another
branchcode (FPL)
6. Save
=> Without this patch the branchcode FPL is used
=> With this patch applied an ugly 500 is returned

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 376bf9b980b877a38acc6680445ad5aa0078620c)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>