Bug 11184: correct attribute cloning for the patron editor

This patch fixes Perl warnings logged when setting up the
patron attribute form in the patron editor.

To test - Patron details entry page:
* Have  ExtendedPatronAttributes enabled. Check that "Additional
  attributes and identifiers" section behaves.
* Verify that editing and saving a patron record does not
  result in the following sorts of entries in the Apache log:

  se of uninitialized value $_ in hash element at memberentry.pl line 798
  Use of uninitialized value in anonymous hash ({}) at memberentry.pl line 798

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Tested with different types of patron attributes:
- repeatable
- linked to an authorized value
- free text
Tested editing, adding, removing one of multiple, adding multiple,
etc. No regressions found.
Passes all tests and QA script.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>