Bug 14185: Undefined $limit causes warn in opac/opac-readingrecord.pl

Bug 14185: Undefined $limit causes warn in opac/opac-readingrecord.pl

This patch sets $limit to be an empty string.

Test plan
=========

1/ login into the opac using your user account credentials
2/ in a terminal, run a `tail -f ` on your instance's opac-error.log
3/ go back to the opac, click on 'your reading history' tab to go to
   opac-readingrecord.pl
4/ notice the warning - "opac-readingrecord.pl: Use of uninitialized
   value $limit" appear in the `tail`ed opac-error.log
5/ apply the patch
6/ reload the page (opac-readingrecord.pl)
7/ page works but the warning in step #4 is no longer logged
8/ run qa test (i.e. koha-qa.pl -c 1 -v 2), there should be no error

Remarks: Testing result match expected test plan output. The QA tests
         pass with "OK" for the commit.

Signed-off-by: Indranil Das Gupta (L2C2 Technologies) <indradg@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>