Bug 26528: Z39.50/SRU ignore invalid replies

Bug 26528: Z39.50/SRU ignore invalid replies

When searching for authorities if an authorities server reply has invalid records
none are displayed.

At least french BNF SRU server doesn't fully follow norm and can return an error
confusing Koha protocol handler which then returns an empty MARC record.

This patch silently removed bogus records.

To Test:
1- Add BNF SRU server
2- Go to authorities page
3- Add an authority
4- Search for keyword(any) droits de l'homme
5- No result (Internal Server Error)
6- Apply patch
7- restart starman
8- redo 4
9- Many records are displayed

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>