Bug 19936: Reuse existing userid if none provided

Bug 19936: Reuse existing userid if none provided

Found this regression when working on other patches:
If you edit a patron and blank the userid field, it will be regenerated
with an incremented value (firstname.surname will be firstname.surname1)

This is because we use a non-existing patron and ->in_storage in
has_valid_userid is always false.
The trick here is to backup the value, generate the userid, then reset
userid to the previous value.

As the POD says, it will be fix later, when AddMember and ModMember will
be replaced with Koha::Patron->store

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>