Bug 28167: Remove uninitialized variable warnings from circ/set-library.pl

Bug 28167: Remove uninitialized variable warnings from circ/set-library.pl

With UseCashRegisters syspref disabled going to the page
/cgi-bin/koha/circ/set-library.pl in staff interface and setting a
library gives the following errors in plack-intranet-error.log:

Use of uninitialized value $register_id in string ne at /kohadevbox/koha/circ/set-library.pl line 79.
Use of uninitialized value $referer in pattern match (m//) at /kohadevbox/koha/circ/set-library.pl line 114.

In the if clause $userenv_register_id appears to be typoed, it should have
been $register_id as $userenv_register_id is always defined. As for the
$referer variable, it is undef if there is no referer so let's just initialize
it to an empty string for the regex so it doesn't give the warning.

To test:
 1) Go directly to /cgi-bin/koha/circ/set-library.pl by typing it in
    the URL bar and set a library
 2) Make sure plack-intranet-error.log doesn't contain the above mentioned errors
    after applying this patch

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>