Bug 18854: Make sure offset will not be < 0 - protect from DoS

Bug 18854: Make sure offset will not be < 0 - protect from DoS

There was a bug that meant a very large offset in the search params
will cause the search script to run forever (or long enough to crash
the machine)

To test

1/ Get ready with sudo top so you can kill the thread before it causes
your machine to OOM
2/ Hit a page like yourdomain.com/cgi-bin/koha/opac-search.pl?q=1&offset=-9999999999999999999
3/ Notice the process runs for a long time
4/ Kill the process
5/ Apply the patch
6/ Hit the page again, notice the it loads (offset is set to zero)
7/ Do the same to search in the staff client

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>