Bug 34723: Show thumbnails on opac-imageviewer.pl when imagenumber not provided

Bug 34723: Show thumbnails on opac-imageviewer.pl when imagenumber not provided

This patch fixes a method call in the opac-imageviewer.pl so that thumbnails
correctly appear.

Test plan:
0. Apply the patch
1. Go to
http://localhost:8081/cgi-bin/koha/admin/preferences.pl?tab=&op=search&searchfield=localcover
2. Change both *LocalCoverImages sysprefs to "Show"
3. Go to
http://localhost:8081/cgi-bin/koha/admin/preferences.pl?tab=&op=search&searchfield=AllowMultipleCovers
4. Change "AllowMultipleCovers" to "Allow"
5. Go to
http://localhost:8081/cgi-bin/koha/catalogue/detail.pl?biblionumber=29
6. Click "Images (0)" tab
7. Download or make two JPEG images
8. Upload the images
9. Go to http://localhost:8080/cgi-bin/koha/opac-imageviewer.pl?biblionumber=29
10. See one large image in the centre and two smaller thumbnails on the right side

PA amended patch: Added missing test plan step

Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>