Bug 17901: Fix possible SQL injection in shelf editing

Bug 17901: Fix possible SQL injection in shelf editing

It has been reported that
/cgi-bin/koha/opac-shelves.pl?op=edit&referer=view&shelfnumber=146&owner=4&shelfname=testX&sortfield=titleaaaaaa\`&category=1

Could lead to SQL injection
Actually it explodes because the generated SQL query is not correctly formated.

However it would be good to limit the possible values for sortfield.

This vulnerability has been reported by MDSec.

Signed-off-by: Mirko Tietgen <mirko@abunchofthings.net>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>