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>

C4/Search.pm

@@ -332,6 +332,7 @@ sub getRecords {
 
     my @servers = @$servers_ref;
     my @sort_by = @$sort_by_ref;
+    $offset = 0 if $offset < 0;
 
     # Initialize variables for the ZOOM connection and results object
     my $zconn;

Koha/SearchEngine/Elasticsearch/Search.pm

@@ -149,6 +149,7 @@ sub search_compat {
     ) = @_;
     my %options;
     $options{offset} = $offset;
+    $offset = 0 if $offset < 0;
     $options{expanded_facet} = $expanded_facet;
     my $results = $self->search($query, undef, $results_per_page, %options);