Bug 14186: Undefined $reservedfor causes warn in opac-reserve.pl

Bug 14186: Undefined $reservedfor causes warn in opac-reserve.pl

This patch sets $reservedfor to an empty string.

Test plan
=========

1/ in a terminal, run `tail -f ` on your instance's opac-error.log
2/ go to the opac and search from an item that exists on the Koha
   instance.
3/ Select the title (if more than one title is returned) and click on
   'Place hold' link to go to opac-reserve.pl
4/ notice the warning - "opac-reserve.pl: Use of uninitialized value
   $reservedfor" appear in the `tail`ed opac-error.log
5/ apply the patch
6/ reload the page (opac-reserve.pl)
7/ page works but the warning in step #4 is no longer thrown up
8/ run qa test (i.e. koha-qa.pl -c 1 -v 2), there should be no error

Remarks: The QA test failed - "forbidden pattern: tab char (line 470)".

         Marking this as 'FAILED QA'

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>