ldb: Ensure not to segfault on a filter such as (mail=)

As reported by Robin McCorkell <xenopathic@gmail.com> triggered by
Mozilla Thunderbird as an LDAP client.

Andrew Bartlett

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Matthieu Patou <mat@matws.net>

Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Sat Jun 22 09:33:14 CEST 2013 on sn-devel-104