Bug 37425: Check for existence of biblio object before fetching cover images

Bug 37425: Check for existence of biblio object before fetching cover images

This patch simply adds a conditional to ensure the biblio object has been retrieved and assumes no cover images otherwise

To test:
 1 - Enable system preference LocalCoverImages
 2 - Perform a search in staff interface
 3 - Find the biblionumebr for one of the results and delete it via the SQL backend:
     DELETE FROM biblio WHERE biblionumber=3;
 4 - Search again.
 5 - KO!
     Can't call method "cover_images" on an undefined value at /usr/share/koha/intranet/cgi-bin/catalogue/search.pl line 671.
 6 - Reindex, confirm error is gone
 7 - Apply patch
 8 - Search again
 9 - Delete a record from the results via SQL
10 - Reload and confirm no error
11 - Reindex and repeat search and confirm no error

Signed-off-by: Roman Dolny <roman.dolny@jezuici.pl>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>